Unlike the PC, where you can install a boot manager in the boot sector, and use it to select an OS and partition to boot into, dual-booting on the Zaurus is not quite so simple. It's also not widely understood. This guide will attempt to answer dual booting questions, and describe various dual booting scenarios.
NOTE THAT THIS DOC IS IN ITS EARLY STAGES, DO ANY OF THIS AT YOUR OWN RISK!
Dual Boot Caveats
It appears that you can only have one kernel in rom at a time, and that kernel must be compatible with every OS/distribution you intend to boot. Dual/Multiboots are obtained by changing the root device just after the kernel boots (in the init process)
Dual Boot Cacko and pdaXrom
In this scenario, Cacko 1.22 is installed as the main rom, and pdaXrom 1.0.0 will be booted if the SD card containing the pdaXrom root image is inserted at boot time. These instructions were written for the C760
It is unknown at this time whether this works on other versions of Cacko, or with other versions of pdaXrom, or on non-C760's.