No

With a little bit of electronics in between, yes.

Wouldn't just be easier to use two button shifters instead, and when needed they can be programmed as a single button shifter..

I know this is obvious for most, but for someone not knowing anything about di2 and stumbling upon this post, what it happens is, they don't just control the rear derailleur, but they will also shift the front derailleur when you drop enough gears, and by long press, you can shift 3 gears iirc

The single-button shifters are meant for full Synchro mode. The buttons control the RD only. So, in synchro, the system (not the shift buttons) automatically shifts the FD for you.
You would have better luck hacking the CAN bus firmware in the battery than designing a CAN bus interface that could emulate eTap FD shifting.

Actually, no hacking required.
One dual button shifter gutted and the chip used in conjunction with the current dual single button setup.
Shifting for up down rear is an OR function and front shifting is an AND function.
Very simple discrete components.
Shimano stuff has no idea it is not a legit four button setup.
Already done it and the bits shrink wrapped and pushed inside of aerobar.

That makes sense; very cool. The way I read your initial post, I though you were creating something new that sits on the CAN bus. That would be pretty hard. But, re-purposing the 2-button guts with 1-button buttons and then rigging the stuff between the buttons and chipset is a whole new angle. Great job!

yes, you put it clearer than I did :)