^^ I think at one point there was a way to hack dual booting on the X5, although its not mentioned in the manual. The OP is probably looking for something similar on the M3.
For the M5 is't actually not a hack anymore at all -- support for generating a dualboot bootloader has been added during DevCon2011. However, Rockbox Utility doesn't have support added, so you need to create the file to flash manually (using mkboot), and I don't think it has been documented in the manual.
As far as I can see from the code this also works for the M3.