The Debian installation instructions give some information about dualbooting.
Although both ARC and SRM can boot Linux, only SRM can boot Tru64 Unix.
It is recommended to install each system on a separate disk.
Use show config in the SRM console to determine disk names and boot $diskname -flags 0 to boot the desired system. This page will be updated if i find a simpler way.

related pages: Linux/alpha, Tru64