My gut feeling would be that this idea is risky for a few reasons -
One is that you just said it - your computer has to find the bootable partition in the USB drive from multiple partitions. Most USB drives I've worked with need to be 1 partition to be read during bootup, and multiple partitions make a mess. Some computers do better with USB booting than others. It might work if the bootable partition is in the first part of the drive space (or is most likely to work that way).
I would also want to shy away from overusing your backup hard drive for multiple functions (loading multiple OS, etc) simply because the idea of a backup is safe data, and everytime you plug that sucker in for other reasons you risk it getting corrupted, and thus risk your data.
The third thing is I notice this drive has 'drag and drop' software already installed, so you'd need to at the very least uninstall that software and reformat the drive. Given the extra software there may be other aspects to the drive that could affect it working.
so, I guess the question is.... Do you feel Lucky? you could learn by doing. (but my gut feeling is keep the USB boot drive and the USB backup drive separate.)