It may be worth your time to review the many hood threads on this forum to gain perspective about this topic. It can be more technically complex then the physical fabrication of a hood. In any case, you are advised to ask your contractor why he is suggesting a pair of blowers. Perhaps he believes that the architecture he is dealing with constrains the duct size to an under-sized sectional area. This often suggests that the entire kitchen layout should be reconsidered if one is in the planning stage.

Normally, one blower is used with sufficient performance that, at the pressure losses that it sees, it can handle the needed flow rate. In some unusual cases, particularly where the ducting or lack of make-up air leads to a lot of pressure loss and a single blower appropriate for this condition would be very noisy, a pair of blowers may be suitable. However ...., there is risk that they will interact with one another, even if each has a monotonic fan curve. It would be an interesting simulation project (which I don't have the time to perform to assuage my curiosity) to see what the conditions would have to be to either cause or avoid such an outcome.

Rather than that, let's go over my recommended requirements that should lead to a single blower solution.

1. Actual full-power airflow rate should be around 90 CFM per square foot of hood entry aperture. (An induction cooktop might get away with somewhat less, due to no combustion aiding the cooking plume temperature. However, this likely only applies if the hood is large enough to overlap more of the plume.) You need to find your hood's interior dimensions at the bottom to determine the aperture size.
2. Blowers do not achieve their rated flow rate except when hanging in free air. Use a multiplier of 1.5 on the result of the first step if you don't have a fan curve and can't estimate all the pressure losses to determine a suitable blower CFM rating.
3. Your ducting should have a velocity of not more than 2000 ft/min. Lower, towards 1000 ft/min is better. Typically, for 1200 CFM blowers, a 10-inch diameter duct is sufficient.
4. You will need a make-up air (MUA) system. See many threads on this topic.

A roof blower will separate the largest source of noise from the cook. A silencer can (if room allows) be inserted in the duct path to make the hood system relatively quiet. An inline blower near the roof with a silencer behind it (hood side) can also work. A blower internal to the hood will be the noisiest solution because there is no way to suppress the blade tip turbulence noise.

Usually, baffle hiss (from lip turbulence) will dominate if a silencer is used, but a constricting duct might dominate under some conditions.

I’d bet that he is telling you that you need code required Make Up Air, But you didn’t understand it. You can’t shove a bunch of air out of a house without a way to replace it. Otherwise, you risk backdrafting down chimneys, water heater flues, and other home penetrations. So you need a controlled source for replacement air that doesn’t let back flow happen. In cold climates, it needs to be heated replacement air. That’s costs about as much as your range package.