Welcome to vintage diving.
Yes, you can modify a double hose to support a BC, octapus or such as that and the easiest way to do that is to save your money and next time there is a release of the PHOENIX conversion kits--PURCHASE one ASAP!!!!!!!!!!!!!!!!!!
Phoenix:
Here is another way using a hookah adapter from Bryan's store and common swivel Tee three way and yes--hurry--get a banjo adapter right away for your spg:
For a first double hose I would get a USD DA Aqua Master or spend the cash and get a Royal Aqua Master.The Phoenix will convert a DA to a Royal BTW.
I can only speak for myself, depth, it depends on what vintage rig I am diving, Essentially the limits are those that are wise for any equipment. When I dive this combination of modern and old gear.
OxyCheq Mach V on Hammerhead tech plate with slung 19 cf pony:
or OxyCheq Mach V on Freedom Plate:
But when I dive something like this then I stay a bit more reasonable:
Here is my old early 80s (non vintage SeaTec wing) with Phoenix and stuff:
I forget how deep I was here but it was a cold water, limited viz dive well in excess of 100 feet--solo:
Here is myself at Weeki Wachee in a mostly vintage outfit, sharskin suit, steel 72, Nemrod manual horsecollar BC, 1958 USD MIstral, Voit Skin Diver fins and old Healthways mask, a perfectly adequate diving outfit for most any normal sport diving. SPG is run from a banjo adapter and is a vintage SeaVue and the octapus---don't need no stink'n octapus!
Your questions have many answers, those or mine more or less.
In the vintage world, when you see something you need--you better get it NOW because things in general or a limited market with small runs of custom reproduction parts that come and go and come and go.
Disclaimer, some of the stuff above is clearly non vintage and some is very vintage, I tend to mix and match or other times stay fully vintage as my needs require.JFYI, the Phoenix rigs above are fully capable of any sport dive one would undertake. Double hose regs are probably not your best bet for serious penetration diving or caves.
Nemrod