There are various ways of booting MeeGo on the N900, and they are documented. However, the way you want, having both Maemo and MeeGo on the eMMC and dual booting between them, isn't documented because, apparently, no one has done it.

There is no guide.

If you don't wish to use any of the documented methods of installing MeeGo, you'll need to figure out your desired method on your own or wait for someone to sometime, maybe, become interested in dual booting from the eMMC and develop a method of making it work.