Moving to a new places always has the possibility of splitting a bond, due to everything being new and not yet claimed. Chasing is a type of aggression, and youve mentioned fur pulling and mounting as well. So although they might not be fighting, it is a clear sign that their bond has broken a bit. Its not uncommon, and usually buns can work it out themselves within a few days. Getting a second litter box is a good idea, and if you feel the need, getting a second cage couldnt hurt just in case either bun decides to escalate into a fight
If they dont work it out themselves in the next few days, you can try separating them for about a week, then trying to bond them again. Since they were previously bonded, bonding them again shouldnt be too hard assuming there have been no bad scuffles. Youll want to start them in something small like a box or the bath tub, then over the course of a few days/weeks get them used to sharing the living room