Using Time Machine with two USB Drives

I'm kind of poor right now, so I can't afford to buy an external harddrive.I happen to have two USB drives that add up to the equivalent of the size of my harddrive.

When I open Time Machine to try to select both, it doesn't let me. Is there a way to select two external devices, and if no, why not?

Time Machine is designed to only work with one external device at a time. You can not choose two devices. If you run out of space on the drive Time Machine will delete the oldest backup to make room for the newest.