I think you're mistaken about the ability of 2 proposers to move to phase 2 - once an acceptor has agreed to a proposal with a certain seq number, it cannot just agree to another one with the same seq number. And that means only one of the two proposers can obtain the required majority of votes to start sending Accept messages, and thus only one of the two values can be accepted by any acceptor.
Maybe read the papers on Paxos, like the "Paxos Made Simple" one, it's not too big (though it's not all that simple either, at least to me...).
Maybe read the papers on Paxos, like the "Paxos Made Simple" one, it's not too big (though it's not all that simple either, at least to me...).