To shoot any Cass except the Three Gun deal, yep you need 2 revolvers. You only need a Derringer for side matches. In 12 years of shooting Cass I've fired a Derringer once.
3 Gun reference is the three styles I:E Revolver, Rifle & Shotgun...in fact your need 2 revolvers or reload on the clock (which is not Gamer speed friendly, the whole idea behind W3G, that & movement)
However, there are
two gun classes in NCOWS, these include Working Cowboy & Orginials.
Some, SASS clubs are begining to offer a two gun regiment of classes , on a limited local level.
OMV to heavy ? well , good many are still used and will be a mighty long time.
I think you would be better served to skip the Bond, and find a good used OMV to pair with yours.