Out of sheer curiosity, I’ve just had a quick go at replicating it on try.discourse.org (that has a 1 emoji in title limit) and I can squeeze in two using the emojis from my phone gallery by not leaving a space as above.
It seems to treat the first one differently to the second for some reason.
I’m afraid I have no technical solution though. Alternatively, could you open up the OP, snip the emojis out of the title, and leave an edit note asking the user to stop circumventing your forum rules? (or a pm to the same effect?)