If the Empire of Nicea never united with the Latin Empire, could both Empires focus on Asia Minor and European threats respectivly? Could this lead to one or both empires lasting longer than 1453?
The Empire of Nicaea conquered the Latin Empire, it did not unite it with it. Not trying to nitpick, just that the two states were rather antagonistic neighbors, and a situation where both survive longer than OTL (meaning primarily that the Latin Empire survives longer) is not going to go smoothly.
It (Nicaea) might have been able to deal with threats in Asia Minor due to paying more attention to the area, but that would require it being able to do so - it facing threats from Europe the same way as the reformed Byzantine Empire did would cost it the same way.
And the Latin Empire was virtually stillborn, as far as being a state with any meaningful power. If not Nicaea, Bulgaria will probably take it out.