Help with regards to managing music libraries between two computers

I have ordered a new Retina MBP to replace my early 2008 MBP, and have been contemplating how to manage my music libraries between both computers. My music library is about 60GB, and I don't want to keep it all on my rMBP (to save space with only 256GB available). My 2008 MBP will serve as a repository for all of my music, and regular Time Machine backups will be carried out. I also have a small, portable external drive which will also have all of my music (copied iTunes folder). The goal here is to have the music libraries on my old MBP and this external drive to be identical, for backup reasons as well as to be able to transfer music to my new rMBP when necessary. To do this, I reasoned out the following method:

-Obtain new music on rMBP

-Transfer folder of respective album(s) to copied iTunes folder in external HD

-Connect external HD to old MBP

-Open music files from external HD (they'll copy into both the iTunes library and the iTunes media folder on the old MBP)

-Verify that the music files are in both places

I can also obrain new music on the old MBP and then transfer it to the external HD.

This seems like somewhat of a cumbersome manner to go about this (first world problems, I know). Does anybody have any suggestions as to how to streamline, or perhaps even automate this process? I was trying to think of a way I could utilize the "Automatically Add to iTunes" folder, but to no avail. Again, my primary goals are as follows:

-To have identical music libraries on my old MBP and the external HD

-To have only a small, can't-do-without-it music library on my rMBP; I could tranfer music over as necessary or play it using the library on the external HD.

Any feedback would be greatly appreciated!

P.