Ok, however I don’t have any experience with this:
Afaik, you can stitch multiple maps together using PatchIMG. Best practice seems to not make the maps too large, otherwise they will be very loaded very slowly it seems.
Lots of info on map making is here: http://www.malsingmaps.com in fhe forums and wiki.
The problem of working with the planet.osm files is that most (if not all) XML parsers want to load the entire file into memory (which will seldom be possible). So most ‘parsers’ are just simple string search cut and past type of applications. These have the tendency to be difficult to function properly. I think this is the reason that there is no proper support for planet.osm files in most OSM utilities unfortunately.