Multichannel mixes are part of another layer so you should add another medium.
Keep current medium as is but change its type to Hybrid SACD (CD layer) and add your 5.1 mixes in a new medium with type Hybrid SACD (SACD layer). 
Also, you could (or even should) use this existing topic below, when some edits need to be voted for whatever reason:
Remember to always explain the reason, like you nicely did here, not just Please vote this edit. We should understand and agree before clicking any links. 