Homepage can be found here: http://opcon.github.io/Cync/
Source code here: https://github.com/opcon/Cync
I've been using rockbox for a long time, originally on my iRiver h10 and currently with my Sansa ClipZip and it is amazing. Unfortunately I have never been completely satisfied with the existing syncing solutions, particularly now that I have two memories available, both the sdcard and the ClipZip's internal memory.
I ended up writing my own syncing application that, among other things, lets you specify multiple destinations which will be filled in sequence.
Cync also copies over playlists and converts them to rockbox-friendly paths. Each destination allows you to specify a playlist root (e.g. '../<microSD1>' which will be appended to the path when each file in that destination is added to a playlist.
Hopefully someone else will find this useful, so that it's not just me haha.