sync master apple lossless itunes library to slave AAC itunes library I prefer to listen to my music on my laptop in Apple Lossless at home. On the go however AAC is fine, and a must, as I don't have enough space on my SSD.

Itunes has the feature to automatically convert AL to ACC when syncing with an ipod. Could this be replicated to work with another mac too? I would then have my master library (with AL) aliased to an external drive I hook up when at home, but when on the go it would fall back to my slave library (AAC) on my local SSD.
This probably can be achieved by just using iTunes Match, don't know if you're willing to subscribe to this. The master library with AL is getting matched, giving you access to the AAC versions from the cloud. From then on, everything you do to a file in your AL library is getting automatically synced to the iCloud AAC version.

When you're on the go just switch to the local SSD library. Start with an empty library, activate iTunes Match and all your music get's visible without being physically on the device. You could then download all the AAC versions from iTunes or just those you want to have on the device. All changes between your master and this slave library should get synced, you just have to manually download music on your SSD when you add something new to your master library.