No, you can't use two 56k modems at the same time. Perhaps there's a way you could get your computer to have two PPP connections going at the same time, but even if you could get it to work it wouldn't improve your speed at all. Besides that, you'd need two phone lines, and two ISP's (since they're going to be upset if they see the same account dialed in twice).
As lowtech said, you can get a duel ISDN setup, but it's not cheap. For the money you spend you might as well get broadband.