Sorry cannot help with any suggestions but find your idea quite interesting. I know
Busch & Muller also do a stem mount but it would suffer the same problem as you have now: a single light, unless it could be hacked in some way.
EDIT: It just occured to me to have a poke around SJS Cycles in the UK for ideas including the following ...
Additive Spacer Two Device Holder
Dia-Compe Gran Compe MH-1 Handlebar Accessory Mount
Thorn Accessory Bar T Shaped 172.5 mm Extension
Mounty Space Bar II Stem Cap Fitting
There are by the looks some other options as well but this should get you started
