Keep your existing reg setup the same, buy a new 1'st/2'nd stage setup for the pony and set it up as a stage (slung) instead of mounting it on your tank (where you can't hand it off to another diver, check for leaks, manipulate the valve, check tank pressure, etc).
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Well as for leaks, I can't check my main tank so I don't see a problem with another back there that I can't see. Valves I can reach just fine as long as you connect the mounts right and not too low, tank pressure is the only thing I can't check. As for handing it off, the pony bottle is for me, myself and I in case "I" run out of air, not my buddy. If for some reason I need to hand the pony off to a buddy, after giving him the reg he can simply swim behind me, pull the cotter pin, take the bottle and off he goes.