Yes, definitely. People can sign on for a primary work and also do one or more complements for other peoples' primary work(s). Sometimes participants get a bit carried away and sign on for too many things (*rolls eyes, points at self*) - that can happen in the comment party stage, which is why it's useful to have that, then to have a chance to take a hard look at what you've squeed about and decide how many of those you can realistically deliver on, before the actual sign-up.