With all the talk of re-engining Antares, many possibilities arise. Solids, new liquid engine, existing Russian engines (RD-180, RD-190), etc.
Focussing on the RD-1xx family though, an RD-180 is a bit overpowered for Antares, but more thrust is rarely a problem, and you need a stage redesign anyway, so roll with it, and boost your performance.
But there was some unsourced speculation that maybe two RD-190's were possible, if ULA managed to lock them out of the RD-180 source.
The RD-170 was a four chamber, but one turbopump engine. Is it one or 4 engines? 1 because although it has four 'bells' it only has one set of turbo-machinery. (Used on Zenit as Sea Launch, and as strap on boosters for Energia)
Split in half, is the RD-180 with 2 bells, and one set of turbomachinery. Used on Atlas V (and Atlas III (now retired)).
Split in half again, is the RD-190 family, with a single bell and single set of turbomachinery. Used on Angara family in Russia.
The thrust scales mostly linerally with the RD-190 having about 1/4 the thrust of the RD-170.
All in all, an immenseley successful family of engines, well done to the designers!
So on a re-engined Antares, would there be a benefit or cost, to using two RD-190s vs a single RD-180.