Dante, There is an LFI issue that discusses two Metz flashes, with full M-TTL functionality using the 3502-M5 SCA adapter. I would have to look the exact types up. It seems to work rather well, the only hitch being that the camera-mounted one won't fire its small secondary flash in GNC mode, and that the bracket-flash will, but is a bit of a pita ergonomically.
Having said that, I get the best results of all with my old Metz Z-2 flash in automatic mode on the 3502-M4 adapter.Unfortunately it is melting down and has not much longer to live, so I am looking for a similar replacement, I have seen an old Metz MZ3.
Philipp - you need a SCA 3000 compatible flash with 3502-M4 or M5 adapter. The 351 won't show flash-ready, nor switch the camera to sync. On the other hand, any flash with a hot shoe will fire, of course.