Was it necessary for the common ancestor with fused chromosome 2 to
mate with another creature whose 2 chromosomes fused in a similar
Of course not. People with balanced translocations have kids with people with the wild-type chromosome arrangement all the time. Such people have some fertility problems, due to problematic meiosis leading to more inviable gametes, but they can still reproduce.
Someone already mentioned Robertsonian translocations to you in your previous question. Didn't you look it up?
would they also need to mate within themselves or could they mate with
the chimp population?
What chimp population? There were no modern chimps when the fusion happened.
Look, you could ask a million questions here, and still learn nothing. This board is helpful for people who have a good, accurate framework of the facts of biology, but who want specific holes in their knowledge base filled. I don't think that's you. I think you think you know the right answers to your questions, based not on science, but on other things, have imbibed or made up a whole slew of very wrong stuff about biology, and are asking questions here expecting people to confirm your wrong beliefs, trying to trick people in to confessing that mainstream biology is wrong, and you are right.
Picking at one bit or another of evolutionary biology is not going to help you. You need to start by assuming that a lot of what you think you know about the scientific facts is wrong, and learn the real stuff. It is not at all easy to get wrong information out of our brains, but until you do, this exercise isn't going to educate you much at all.