Adding five of a kind also seems like a pretty easy adjustment.
The most obvious difference is the possibility of cards matching both suit and rank. So if you have HA HK and the flop is HA HJ S2 you have both top pair and also a very strong flush draw (although HA HA would be better), a combo which is impossible on that board with a single deck.
The issue is that this is a completely different game. It might be a fine game. People might like it (maybe even more than Texas Hold 'em). But this isn't the same game anymore. They're both played with the same 52 data points, but the frequency of those 52 data points makes these games and the skills needed to be good at them very different.
My previous comment of the game being "unplayable" might have been hyperbolic. The point I was trying to make is that it's not the same game when you change one of the rules so dramatically.