no

I am being told that the 12/3 wire can be split into 2 separate 20 amp circuits via 2 breakers together in the panel.

HU, check your facts. The electrician isn't installing a single wire, they are installing a single cable. Yes, it is physically possible to wire 2 separate circuits with 12/3 cable.

OP, are you sure that the electrician intends to power the dishwasher, disposal, and built-in microwave with this same cable? Those appliances should all be on their own dedicated circuits. Then you need two separate circuits for the receptacles.

The two HU's are incorrect. A 12/3 cable can be installed on two breakers, with the breakers on opposite legs of the 240V line.

Catbuilder - thank you for clarifying. Yes, there is only one 12/3 cable/wire to power all of the items - dishwasher, microwave, garbage disposal, and 2 outlets. This seems like a lot of power needs for this single 12/3 wire/cable.

It's two circuits, but it's not to code as all built-in appliances should have their own dedicated circuit. Are you having it inspected? Is the refrigerator on a separate circuit? Do you have other receptacles on circuits other than these?

Why is your electrician not providing separate circuits for these appliances?

All great questions - refrigerator, oven, vent hood, and other counter receptacles all of separate circuits/lines on the back wall away from the island. Honestly, I don’t know why only this single 12/3 cable was run for all of this as the slab was cut open to move plumbing lines as well as run the cable. All walls/ceiling were open as well. Cabinet guy made a comment about it, then I called GC who said he called his electrician and electrician said 12/3 on 2 circuits was sufficient for those appliances.

Also, my city is on the 2014 NEC. They have already inspected electrical in order to close in the walls.

If they've already inspected it, then what exactly are you asking? The inspector says it's fine, the electrician says it's fine. These are people you know. You don't know random strangers on the internet.

The misunderstanding by both HU's comments (BTW same guy) is that a 12/3 cable will have black, red, white, + ground. The black and red will be separate circuits with a shared neutral. This is allowed when both breakers are tied, so that shutting off one will shut off the other simultaneously.

Assuming neither circuit is a SABC. Whether it is allowed does not mean it is a preferable method. The DW circuit will be GFCI, and really should be a plug-in cord under the sink to follow the 2020 NEC (soon to be approved by your city). If the DW, microwave, and disposal share a circuit those are power hungry beasts that should have their own (at least DW and micro). If in the slab, you should ask (and pay for) another cable that will fit in the conduit (may have to go to THHN).

You can have a single feed to two separate circuits but may cause complications with the requirement of Arc Fault interrupters.

NEC requires two dedicated small appliance circuits as well as dedicated appliance circuits...Your kitchen is not being built to code...curious what code cycle your municipality has adopted.

As an update, I have reached out to the City's Electrical Inspector and awaiting a call back. I will post back when I get a definitive answer. Thanks for all responses.

In principle, the 3 wire plus ground cable could be fed by a simple 240 Vac breaker and would, in turn, feed a small breaker box at the island. This breaker box would be populated with breakers tailored to the requirements of the particular circuits that they fed. It would be desirable for the feed to have been heavier gauge because I am not sure that the load will never be greater than 80% of the feed cable's ampacity without seeing all the specified loads. And since the island itself requires a receptacle (pile of relevant rules incorporated by reference) that could be loaded by a high current plug-in portable appliance. meeting code might be a challenge.

If that cable can be pulled from the conduit and there is conduit to boxes at each end, then the conduit could be repopulated by lower AWG number THWN to feed a suitable breaker box (perhaps as few as six breakers). You want no limitations on what can run simultaneously, and also voltage drop minimized for all combinations of activity.

I assume that you have additional wall outlets in this kitchen fed separately.

Thanks everyone - after much discussion, GC has agreed to run additional dedicated circuits to island appliances and outlets