There is no easy to use noob guide to dual boot between Maemo and MeeGo on the N900 yet, but if you want a sneak peak at the signs of things to come, Carsten Munk (@stskeeps) has put together a brief demo showing the N900 booting between Maemo and MeeGo pretty seamlessly.
Carsten is the official maintainer of the MeeGo Nokia N900 hardware adaptation and goes on to say:
MeeGo 1.1 isn’t out yet, give us a little bit of time. What will happen if things go right:
* Package install from extras-devel that enables the u-boot boot loader.
* Upon startup, if there is a SD card that’s bootable, it will boot the SD card, with MeeGo. Otherwise, it will boot Maemo.
More customized setups are possible for the daring.
MeeGo will not come to the N900 officially i.e. Nokia will not provide it via the Nokia Software Updater or any other sort of download. However, like I have said before, there will be a community based version which you will be able to install by following a brief tutorial. The pieces are almost in place, its only a matter of time.