In the People/Pets view I dragged one picture over another and got this:
and they merged properly. The way the little thumbnails bounce around apparently chaotically seems awkward, but the merging worked. Then I dragged the result to another one, and that worked, too. (I made alternate names for the person so I'd get additional albums without messing up the original.)
I think I remember that i couldn't put pets with people, and vice versa, but otherwise it seems OK. I'm in Sequoia 10.1.1, but I know I had it work in 10.0.
You said "collection," but I assumed you meant the people albums--not Trips or something that are now called "Collections."
What do you think we did differently?