It'd be tricky, but could you do a hard mode where each guess must include all valid letters in one or both puzzles, like in original wordle? I always play that way, but it'd be fun to have it enforced.

For, you know, certain values of "fun."

Just my $0.02 but I don't think hard mode translates well to Dordle, since it's literally impossible to play it that way on both sides simultaneously, and "on one side or the other" seems pretty arbitrary. Wordle is pretty easy even in hard mode, but Dordle's already hard enough!! :)

Started playing this way after reading your comment, and it's honestly much more rewarding.  +1 to this suggestion, as it would be really nice to have the game check for me, so I don't accidentally submit a word that isn't valid for the current clues.

How would hard mode logic work in dordle?
Let's work an example. Suppose (3 letter words to keep it simple) the secret words are DOG and CAT. Suppose your first guess is COD, which gets played on both sides. On the left side (the DOG side), O gets a green and D gets a yellow. On the right side (the CAT side), C gets a green. So now what words are allowed? Must you have C first (since it got a green on the right) and O second (since it got a green on the left)? Must it have a D somewhere in it, since D got a yellow? If all of those are required then you're stuck playing COD again and can never solve the puzzle. If only some are required, then you can only ever solve one side. Is there different logic that makes it work? I'm not sure there is.

no no, it can totally work if you use an OR. 

You can play, say CAR, which is a valid hard mode move for one side, OR you can play something that's a valid hard mode move like DOE for the other side. As long as your word is a hard mode move for one side or the other, it's allowed.

I've been playing this way and it's definitely possible to win. I've only lost a handful of times, in fact.