Podge has tummy issues, and produces an amazing variety of poop sizes, shapes and textures, I can tell his poops from Ginas no problem, so if not telling poops apart is the only reason for not bonding, I think you could be confident to tell two bunnies poops apart. Plus any decrease in double the amount of poops is just as clear as a decrease in a single bunnies-worth of poops. And even with three pairs, it is always very clear if one bun is off their food, they don’t rush over when the pellets and greens arrive, and they don’t take their favourite treats when held out to them. There is the other side that having a bun partner can help a bun through feeling poorly. I don’t think Jenny would still be here today if I hadn’t been able to bond her to Pippi.
Bringing another bun in is always going to change the status quo. When I adopted our third pair, It took a while, but for all they are territorial towards each other, I think they still get more out of the extra bun company than they’d ever willingly let on!