Thanks for your answer Reader50
What are you trying for exactly? For each MBP to backup to a different disk? Or for both MBPs to alternate, so you have duplicate TM backups?
Lets say Disk 1 (D1) and disk 2 (D) are connected to the MM in the network.
D1 and D2 each have a partition that MBP1 and MBP2 backup to individually (MBP1 to D1 and D2, MBP2 to D1 and D2).
As TM is capable (when the disks are directly connected to the MBP) to backup to two disks alternating between them.. hour one to D1, hour two to D2, hour three to D1, etc.
This is what I like to achieve while D1 and D2 are connected to the MM in the network. Am I clear here?
If the first case, mount both external HDs on your Mac Mini. On each MBP, select the shared disc intended for that notebook.
No not what I want.
If the 2nd case, it's probably easiest to format the externals on the Mac Mini as a single RAID-1 volume. Share that volume, point both notebooks to it.
If this indeed the solution to my problem I do not know how to do that. However I do not think it is what I want.