Keeping basically two different browser windows in sync with the same data and/or user interaction. The two windows can’t communicate to each other without bouncing data off an EBS, and you’d have to add error checking to ensure Window A captured a UI change that was initiated in Window B and Window B itself actually capture the response itself.
At least the issue of the code know which it was is straight forward but ensuring sanity between the two windows without just hail mary-ing it would be fun. You don’t generally see this on the internet anywhere, can’t think of a recent example that does.