Actually just one more question if I may....

Would we only need new disks for the Data and Logs for the new instance? (and just keep the single instances of Quorum and MSDTC for the whole cluster)

So we would go from this:

Q: - Quorum

M: - MSDTC

D: - Data

L: - Logs

...to this:

Q: - Quorum

M: - MSDTC

D: - Instance1 Data

L: - Instance1 Logs

K: - Instance2 Data

J: - Instance2 Logs

(...we won't worry where the tempdb is for now! ;-))