I'm writing this reply on a multilinked modem computer.
1st, your ISP has to support multilinking. You usually have to sign up for 2 accts. to do this. My ISP (PCU.Net) gave me a discount on the second acct for a total of $20/ month.
2nd, you need two SEPERATE phone lines from your phone co.
3rd, in WinXP, go to connect to > show all connections > right-click on your ISP > properties > networking > settings > and check the "negotiate multilink for single link" connections box.
It makes 56k surfing and downloading more tolerable, but don't expect better online game performance (lower ping) in fact mine got a little worse.
EDIT... it sounds like you're having a hardware conflict as well. I'm using a External serial modem (OD EMS-100) combined w/ a Internal PCI modem (USR 5610B)