I haven't tried what you are asking, but I do lots of iPod syncing so am taking educated guesses:
>> Q1: Is it possible to sync multiple libraries to an iPod?
Probably yes, if the sync process is set to NOT delete anything already on the iPod.
>> Q2: Is it possible to create a playlist containing music from multiple libraries?
Probably no, since a playlist can't "see" another library, which is usually physically totally separate. (A library is actually a bunch of files that MC combines when the library is loaded.)
Which raises the question, why are multiple libraries needed? Using fields/tags, especially the all-powerful Keywords field, and perhaps some custom fields, it is quite feasible to have one library that contains every possible type of music, then use views and smartlists to create virtual libraries to work with, listen to, and copy onto an iPod.
My main library is more than 80 thousand tracks of almost everything -- rock, pop, rb, jazz but also classical, showtunes, big band... every musical genre. Whatever music I have gets tossed in the library, which is called "Music". The critical ingredient is effective tagging, a one-time-per-track task that has huge payback.
The only separate libraries I have are for non-music audio (radio shows, for instance), for photos, and for video.