Hi,
Unless if you're sure of the second guy, he could still say yes, "I go for A and you'll go for B " and in the end, he might try to outbid you on B as well, or
you may get outbid on B by a third fella and get p*ssed if the other one got the auction A.
Ebay is public, I don't see how you can know all the bidders before they bid as some have the "bid on the last 2 minutes before end of auction" strategy .
Fix an amount that you don't want to go over and keep your bids under that amount.
Whatever you bid, you can loose the auction .
That is the basics about bidding .
If you know well the other bidder, try to talk to him BUT keep it private, and don't make waves if he gets pissed from your contacting him and hope he won't make waves.
Don't insist if he doesn't answer your emails .
... you should perhaps delete this topic :) .
And paddawan, my only advice will be: "bid on the last 2 minutes before end of auction"
Good luck !