In Firefox it is possible to set multiple homepages using the | symbol. It is also possible to allow a user to have multiple homepages stored locally using this setting. Firefox Portable however currently only loads one local homepage, the rest it interperates as an http: address. Just a small gripe there's probably just a handful of users that would use multiple tiddly wikis' and whatnot.
Firefox Portable *is* Firefox. The copy of Firefox inside is unaltered. You can set multiple homepages the exact same way.
Now, as you noticed, local homepages are an entirely different thing. Since your drive letter changes, you can't set a local homepage in Firefox itself. I added the ability for the launcher to set a new local homepage for you on every launch for people that want to use something like TiddlyWiki. It only works with a single local file since it has to figure out the full path to whatever you want and then manually add that setting to the prefs.js on every. It won't be updated to support multiple with a local file. Why not create your own HTML file that links to both of them (relative links of course) or puts em both in a frame?
Sometimes, the impossible can become possible, if you're awesome!