(a) if they jump together

(b) if they jump one after another

(c) which velocity is greater?

I followed the path of using equal total momentum before and after to solve the problem, and I just but for some reason, my (a) velocity expression is greater than my (b) velocity expression. Is this supposed to happen? I assumed this is like a multi-stage system, where final velocity should be higher if there are more stages.

My set up is (M+2m)vi = 2m(vi - u) + M(vf) where vi is initial velocity of boat and vf is final (what im looking for)

(vi - u) would be velocity of the two people relative to water.. right?