One problem with using a BOB is the need to verify its working (not flooded) and or keep it active during the dive.
I have found that the need to remove the primary dsv from the mouth and insert the bob one to be enough of a pain to do during busy dives that Im not encouraged to do so, also far more importantly (as I learnt the hard way) taking a perfectly working dsv out of your mouth can add problems/risk if you flood your primary (or bob) in the process.
So I looked at the design of a dsv that houses both loop dsvs.
trouble is they become too big and heavy and unpractical.
Then I hit on a very very simple solution.....
Its a bit of a cheat but here it is
1) Use a Panorma Nova FFM
2) Primary DSV goes in mask front p-port as normal
3) BOB dsv* goes into one of the side P-ports in mask
During the dive to verify the bob is working I just need to reach up and open the BOB dsv lever and nose breathe.
If I need to bail I just have to move the BOB dsv from the side p-port to the front p-port and insert a blanking plug (secured to side of mask with bit of cave line) or blank it with the primary dsv (harder but possible)
[Tipping head to side stops water ingress through side p-port when swapping dsvs ]
Another trick that works really well and totally prevents accidental floods is to use the BOB CL as part of bouyancy. During descent I alternate adding gas manually to both wing as well as BOB.
Used this on the Song Hong exped at it worked like a charm. With the ADV combined with using the bob as part of bouyancy the BOB loop was +ve the whole time during the dives helping to keep water out
*(note: must seal off BOB dsv water vent hole)