USB C to Dual HDMI working as two independent extended Monitors

I would question the adapter.

My guess is that that is NOT a Thunderbolt adapter, and as such, on a Mac it is just seeing 1 set of DisplayPort signals, so at best it can just mirror the 2 HDMI ports.

Chances are this adapter is intended for Windows, and not a Mac.

If you want multiple monitors from a single USB-C connector on a Mac, you MUST use a Thunderbolt dock. The adapter MUST say Thunderbolt in the description, otherwise it is just a USB dock, and at best it has access to a single DisplayPort set of signals.

Barney and Bob are almost certainly correct, but to make sure: what do you see in System Settings->Displays?

One or two external displays? If it shows only one, then there is nothing one can do but follow their advice.

If it shows two displays, then you may just have to change the settings there...

Can you post a screenshot of System Settings->Displays, something like the image below; here I have my iPad mirroring my external display, just as an example, but I still can discern the two and change them to be independent displays. If you only see one extra display, then no dice.

I don't think it is possible. You'd need a "dual HDMI" converter that sends two different video signals.

But, I can't imagine how one port could send out different video signals.

Does your dual HDMI converter say it can send two different signals, or does it just split the signal?