One of my learners is progressing substantially faster than the other and is ready for buttons to be moved to one place. He's also ready for more buttons. Should I slow him down and let my other learner catch up? If I go ahead and consolidate buttons how will that affect my slower learner? Any advice appreciated.
Leo ’s advice is to progress at the speed of your faster learner. One thing we did (not deliberately, it just sorta happened as we were slowly moving everything) was consolidate buttons into related clusters. We had places closer to the door, social words by the couch and actions in the middle of the room. Close enough that we could reach them all to model a phrase like Hungry Now Yes, or Walk Outside but not one giant soundboard. That might be a good intermediate stage for you guys to hang out in.
I'm also wondering the same thing for my two.
I have two dogs - Kenji & Shiro. Kenji has picked up the button right away and is using it accurately. Shiro has little interest at this time. Should I be adding words to work at Kenji's pace or should I slow down and wait for Shiro to catch on?
I have two separate areas. Β Luckily, what motivates my one, my other doesn’t care about. Β So I can add buttons to my one who’s really liking it and continue to encourage my other who really has no use for this silly human game. Β 
Our household just became a 2 dog household, I definitely want to do the buttons with Nori, and will model it for him all the time. How does it affect him if the 2nd dog doesn't have to go through that? Our roommate only wants to use the potty button for house training reasons. They are being kept separately for now because my younger pup seems scared of bigger dogs, but we do hope that they will be friends in the long run and we can have them in the same room.Β 
I am working with two. Β Completely different personalities and motivations. Β They have both learned β€œeat,” though my one is far quicker about it. Β Luckily, they both have different interests so they get to work with different buttons.Β