Blloomin Eck!
Did he set it up to return to petrol at low rpm?

Where I've fitted OMVL's on a Merc V8 I've usually made a bracket to mount both rails on top of the engine with outlets pointing outwards, they sit in the space in the middle of the engine cover then.. Surely not fitted by a pro?
Although having said, I have put right quite a few vehicles converted by pros that have two reducers and no balance pipe. Most of them with Stefanelli systems (not even a vac connection on second reducer), but two that spring to mind were a Jag XJ with Bigas system, twin reducer (also needed AEB124s which it didn't have).... don't know who fitted that one. And a Jeep SRT with 2 x OMVL reducers fitted by a well known Autogas place in Yorkshire somewhere around the East coast.. Apparently, the Jag owner thought it was normal that the Jag would run rough on LPG and had put up with it running like that for many years. The Jeep owner had been back to this Yorkshire Autogas place many times complaining of poor running and engine check light problems but they hadn't sorted the problem despite all the return visits.
I once had a DIY customer mount a reducer to the engine on one of these, told him he'd better mount it properly or I wouldn't set it up or have anything to do with it. He ended up asking me to move it and paid me to do so... Dunno why, he fitted the rest of the system fine including the injectors and was easy enough to move the reducer.
Simon
www.Lpgc.co.uk