I am coating a pine mantlepiece with gloss polyurethane. I used interior/exterior poly for my 1st & second coats for protection from sun exposure. Because of the longer drying time 24/48 hours there are quite a bit of dust nibs. Can I use a faster drying interior poly (2 hours) as my top coat or are the two types incompatible?
Since it has already been a day since your last coat coat, the polyurethane should be done drying (along with some curing) so you can safely apply another coat of polyurethane if it has the same base (oil/water).
There might be some issues if you apply different polyurethane (with the same base) within 2 hours since there will be chemical bonding between the two layers but that seems unlikely.
If more than 2 hours have passed but less than a day has gone by you should also avoid new coats since the difference in curing between coats might cause some weird patters to show up once it's all done curing.
If you are mixing between bases (oil/water) ideally you would wait a month, or more. That way you can be sure that it's done curing and won't have any interactions between the different polyurethane layers. Since it's been a few days you might be able to get away with just applying the new coat after some light sanding with 220 sandpaper (I would do this anyway to knock down those dust spots). However, there is chance you might get a weird texture/cracking in the finish.