I’d ask them to have both bunnies meet in an enclosed room, as far away from dogs/cats as possible. It would skew their meeting if one or both is afraid of the sounds of the other animals, and definitely won’t help if they can see cats or dogs. Don’t let them touch unless you are certain at least one is speutered. Don’t worry about a small amount of humping if they are “safe” and can’t get pregnant. However, never let either one try to hump the other’s head and face. Very dangerous in several ways. Also, watch for teeth, biting, and listen for growls. Separate them if the shelter people are not experienced with bunny dating in that case. Be ready to grab your own bunny while they grab theirs. A towel to toss over one bunny can help to stop a fight and let bunny be scooped up. Also, putting a strainer like you use in a kitchen for noodles with a handle can be dropped between their faces before they attack if growling starts and you can’t catch them quickly enough.
Indifference, bored looking, lying down and ignoring – these are all good signs for possible bonding in bunny behavior. If one grooms another and they allow it or switch and seem to like it, that’s very rare and very good.