You could luck out and they could just be very interested in each other with no poop or urine marking going on. But more commonly, if the new bun is sexually mature (2+ months old) and he thinks your bun is a potential mate, even though he's a neutered male, this could cause some hormonal spraying from the new bun. If this happens, blocking him from being able to see your bun, may help with that. A piece of cardboard between pens can work. In the very least, a solid barrier can help contain where the pee gets sprayed.
You really won't know until you get him there and he starts to settle in. But hormonal spraying is one very good reason why people get their pet male buns neutered. It's especially disgusting when you get a shot of that pee right in your face
If you don't have something solid blocking them from seeing each other, do at least make sure there's a gap between their pens, at least until you've determined there's zero aggression from either bun. You just don't want to end up with a bite wound on a nose or ear, especially one that requires medical attention.
See how it goes. As you get a better idea from their reactions to one another, that can help guide you to how you'll need to adapt things to work out for you. That might include moving litter box locations or setting up secondary litter boxes in areas that are getting urine marking puddles. The pen wall between two buns areas can be a common place for this to happen. So having litter boxes directly opposite one another, is often what works best.
With the transition feed, maybe if you offer the person a few dollars, they may be willing to give you some. Saves you having to buy a whole bag.
