Very nice item. It's really unfortunate that multisigned items really do not get the same sort of financial love that single signed items do. I have a great number of multisigned items including many HOF yearbooks/programs similar to yours and love them. The problem is that you really don't get back your per auto investment when you go to sell. That's kind of the rub with adding more to it.
Even though you will not increase the value proportionally by adding extra players, I just love this type of stuff, so I would definitely add to it if I could
safely. Once completed, I think they would look very nice framed up side by side. Sort of like these 2 magazines I have.