Ben,
Since you have two PowerBook computers of the same model, an inexpensive PC Card adapter for CompactFlash could be a useful investment. A CF card is normally a good way of transferring files, and can be used for backup purposes. As indicated earlier, the computer may even be able to boot from a Mac-formatted CF card with a suitable system folder, something that would be of interest here.
Alternatively, you may want to test whether the hard drive moved from the second PowerBook (with its already installed operating system) would allow the first machine to boot. Of course, there is always a certain risk involved with this; any faulty circuits/components inside may damage the hard drive. See pages 75-84 in the manual (link above) for hard drive removal instructions.
Jan