Yes. Consult with the manufacturer.

Yes. Are you seeing it isn’t pulling enough? 10 is much less then 2 -8”. Not sure if adding fan further up line would help or not. Surely wasn’t planned for properly or installed correctly. Exhaust fans are much more tricky than most assume. Probably one of the GCs on here can provide more expertise on this.

At 40” above the range, it doesn’t move smoke at range height. I was expecting more from 1300CFM

Do your have a makeup air damper?  Have you tried opening a window nearby and running the hood fan to see if it performs any better?  While one 10" duct is not what was spec'd, it is still plenty big and I'm surprised to hear that it underperforms that badly.  I'm wondering if there isn't sufficient fresh air coming in to the house.

Ive run it with a door open. Could it be an insufficient roof cap?

It probably could be the roof cap. What style and size do you have?  I've noticed that some of the accessories sold by manufacturers, such as the Zephyr 10" roof cap, seem way too small for the amount of airflow. If it's smaller in area than the 10" diameter duct,  then it could be restricting airflow.

" it says this dual blower unit needs two 8” ducts, but there is only one 10” duct leaving the unit. Could this be the cause of the underperformance? "

That is where I would start. Why is there only 1 10" duct?

Are you sure there is not a transition piece? My Vent-a-hood has a transition piece that has two 8 inch ducts that transitions to one 12 inch. You can see the piece on top of the hood in the picture.

Uninstall, and install the 2 needed ducts. As a start. Then install the code required active Make Up Air system. Do not just crack open a door.

A tour of hood threads on this forum is recommended to the OP. In particular, keep in mind that the hood system flow rate is affected by all pressure losses in the hood air path loop. This includes the baffles or other filter, hood interior transitions, ducting, roof cap, make-up air (MUA) intake and ducting pressure losses, MUA filter loss, MUA heater loss (if present) and register/diffuser loss. Pressure losses reduce blower performance (see generic diagram -- click to enlarge). With proper MUA and all other ducting meeting requirements, achieved CFM is unlikely to be higher than about 2/3 of the rated CFM, which is determined with the blower(s) hanging in free air.

Furthermore, hoods depend on the cooking plumes' upward velocities to rise to the hood, where they are captured by the overlap, and contained by entraining the plumes into the hood air flow. The air velocity at a distance from a hood is very slight, so moving the plumes at the cooking height will be limited to low momentum/low velocity components such as steam from boiling. One wants to have the system capable of capture and containment of high velocity oil/grease at the smoke point (up to 1.2 m/s).

Does one 10” duct equal two 8’s?

Area of a 10" circle is 78.540 square inches.

Area of two 8" circles is 100.53 square inches.

(someone check my math)

How would it? Simple math tells you 2 - 8" would carry move volumn than a single 10".

Without even accounting for the pi^2/4 factor, two 8s squared is 2 x 8^2 = 128 and one 10^2 = 100. Better is a 12-inch providing 12^2 = 144. On the other hand, two 8s into a 10 won't matter much if the hood is gagged by there not being a low pressure loss path for adequate MUA. Otherwise, this rated CFM, even with a factor of 2 assumed penalty, should provide the desirable 90 CFM/sq. ft. (90 ft/min) into the hood base.

Are these blowers common centrifugal fan types, or axial types, or are they squirrel-cage types like VaH Magic Lung blowers. The latter are more easily restricted by pressure losses (fan curve shown above is more horizontal). I'm not going to hunt for the fan curve for this hood, but if anyone notices a curve or list of CFM vs. pressure loss, feel free to direct my attention to it for further comment.

I also recommend the OP review MUA-related threads here to understand not just the code, but the basis for the requirement beyond making the hood work as intended. Hint: whether a blower is needed in the MUA path depends on the situation w.r.t. combustion appliances in the residence and how they are provided air.

Mark is correct and a 12" duct has an area of 113 sq in

Update: after getting in the attic, i saw they immediately took an 80° turn. the manufacturer said this was a big no no. i am going to have them remove the converter and shoot two 8” ducts straight up and out

You're good at math, Mark :)

You may still need to evaluate your MUA.

No make up air

In one sense there is always MUA, because no air goes up the hood duct(s) that isn't replaced in the kitchen. This air may be provided by a deliberate MUA system (possibly comprising only a duct, heater, filter and exit register/diffuser), or depend on house leakage. The latter will very likely be insufficient for the hood to flow even 2/3 its rating, and may cause back-drafting of any combustion appliances taking air from the house volume. At the very least dust will be pulled from the cracks in the walls, and moisture into the walls from outside. These issues need to be understood and addressed for health and safety reasons.

MUA is generally code required and also generally code ignored. The penalty is dependent on the tightness of the house and presence of combustion appliances. Combustion appliances can make it dangerous.

Was this done by a contractor (sounds like it) and were permits pulled? You might be able to make the argument that it is a code violation and it should be fixed on the contractor's dime. Pretty tough argument but it is possible.

MUA done correctly is quite expensive. To truly do it right, you need the same volume blower with the same flow rate. So it basically doubles the cost and you have to have a register somewhere to bring the air in. And dampers to seal it and wires to control the auxiliary motor. See why it is ignored so often?

One cheap out (which I have) is to put a motorized fresh air damper on the HVAC. It opens with the hood and runs the HVAC fan. Depending on size of HVAC, you get some MUA. I think for me it is 800 CFM - with all the caveats about duct size limiting flow. But my fresh air is 10 inch and short/straight.

You've several problems:

Even a single 12" that is larger volume than two 8" will likely not perform as well as two 8". This due to turbulent back pressure (which occurs any time two forced air streams come together). Realistically you'd need a single 14" if you want to combine them.

You do need 1200 CFM mechanical MUA. Passive will not be sufficient unless you live in Hawaii or elsewhere with windows consistently open all year.

This hood has zero containment area so even with proper ducting and MUA it will not perform well with any bursty cooking.

It likely has very weak blowers (steep fan curve) so any bends or caps (static pressure) will dramaticallly reduce performance.

I'm guessing it is quite loud given other problems.

If you want good performance read: https://bamasotan.us/range-exhaust-hood-faq/