Single first stage on twinset?

[Celt]

Yeah definitely possible. Dived a single first and single second for 20 years. Non isolating twin bar. Valve on one side and a valve in the middle. Your using both tanks irrespective of where you mount the reg.

 

[Degenerate]

Or you can just put a short inflator hose on that first stage and plug it into your BCD or SMB after the dive to purge the first stage...?

Or even better, tell your mate to pony up the 150ish dollars for a used reg set and you can keep diving with both of yours.
If you give one of your regs to your mate, would you both just have one second stage each?
Doesn't sound like a superb idea IMO.

 

[rjack321]

You mean the HP port plug right?

If you completely fill all the ports on a junk reg and put it on the left post of your doubles as a giant plug...
It doesnt matter which port plug you loosen when you want to remove it. Any of them will vent the gas behind

 

[Stoo]

Never leave a valve without a reg on it.

This. If that other valve is bumped, you're going to lose a whack of gas in a short period of time.

Others have suggesting your buddy buy a reg. He can also rent one for what, $20 or something?

(Your buddy is certified, right?)