I have created a bootable USB pen drive with SLAX using UNetbootin.
Now I want to also have PartedMagic and SystemRescueCD on the same pen drive as multi-boot, is this possible.
I have ISO's of both but I need detailed instructions as I couldn't figure out myself from the many Google search results.
The pen drive is 2Gb so space should not be a problem.

Hmmm good one.

You would need to mess with the syslinux bootloader. Not too hard.
Have a search on the pendrive for syslinux, or maybe it is using grub. But that's the first step, to be able to edit the bootloader to add more boot options.

Then once you sort that, you need to get the other distro's on there.
Off the top of my head I would suggest installing the other distro's on their own USB drive with UNetbootin and then copy that entire FS off onto your main USB drive, perhaps putting in its own folder. All depends on the first though really.

Just thinking out load here...
Anyone else?