Just so you know:
- OS 9 will only see one CPU at the OS level. There are some applications which will take advantage of the second CPU.
- OS X can be installed on a G2 machine, using XPostFacto - but XPostFacto doesn't have dual 604 support.
- AFAIK, most if not all PPC unix-a-likes will use both CPUs.
In theory, it should be possible to run two instances of MacOnLinux, but I don't know if anyone has tried this.
[*]BeOS PPC has multi-processor support, I think.