You have two Mac’s. You have two iTunes libraries. You never have all your songs on each Mac…
syncOtunes from OLIGROB Software synchronizes your iTunes libraries among various Macs. Just select the iTunes library file from your first Mac. Then open the iTunes library of your second Mac through a network share. syncOtunes will compare the tracks in both iTunes libraries and creates a folder for each of your Mac’s, containing the missing song files. Then simply mport that folder into iTunes with an applescript, unneeded files are automatically trashed, and “poof” both of your Macs’ iTunes libraries are synced.
syncOtunes is freeware for Mac nd there is also a Windows beta available. More info and download link here.