The dual plug FA-120 where supposed to have the inlet/outlet check valve pump & (bleed off) regulator to scavenge the crankcase & atomize the fuel for more power. The dual plugs where supposed to aid in more complete burn. Think FA-300TTDP (Dual Plug Pumped) in a single configuration. I had run across some propaganda about that engine a whils back but can't seem to locate it.
All that being said, I'm not sure it helps that much on a single. On something like the FA-450R3 & FA-200Ti where A/F imbalance between cylinders is a real significant issue, it helps. Then again, CDI helps even more.
If it where me, I would screw a dead plug in the front hole & try running a single plug. The 150s & 180s do just fine on a single plug.