Yes, you can, but with some notes. Braby sent some data, I add some other.
Target Mode has two basic roles:
1. to turn a mac into an external drive and/or external optical drive (if it has one, until Air era, all had); you may use this way if your internal optical drive is damaged and you wish to use the optical unit of the other mac; or use it as a simple external disk for backup purposes or else.
2. to boot from a such-booted mac.
#1 is usable with any two macs disregarding whether they are both PPC, both intel, or combination PPC-intel, as the behavior is like any external disk drive or external optical drive.
#2 is impossible if you want to boot the other mac, they must be both intel or PPC, as you cannot use an intel-based mac as a boot mac for the other ppc-mac, and vice-versa.
So said and clarified, you may use, but not if you intend to boot the other one if you wish to use it for various purposes implying boot.