i don't know of any way to split your iPhoto library. but you certainly can split your iTunes lib.
however, in your case i would start with moving both libs to the external.
the instructions for moving your iPhoto library are
here.
moving your iTunes library to external works similar. copy the
entire iTunes folder (not just the iTunes
media folder) to the external. launch iTunes while pressing
option and click on choose library when prompted. select the iTunes folder you moved to the external.
when the MBA arrives, repeat the
option-launch procedure. you will need to authorize iTunes on the MBA for your iTunes store account.
with this kind of setup, it is important to remember that the drive holding the content needs to be mounted on the desktop
before launching either iPhoto or iTunes ! also, only one instance of iTunes @ a time can access your library.
now, in order to move selected content from the external to your MBA, do this:
(a) launch iTunes on the MBA and go preferences > advanced.
(b) temporarily point
iTunes media folder location to <MacintoshHD>/users/<yourname>/music/iTunes. on the same tab, make sure there are ticks in the little boxes next to
keep iTunes media folder organized and
copy files to ... when adding to library.
(c) select (highlight) the podcasts or any content, right-click (or ctrl-click) on it, and choose
consolidate files from the pop-up menu. this will
copy the selection to your internal HD so you might want to delete the
duplicates from the external.
(d) when done, be sure to point
iTunes media folder location back to the external.
