757-767
Actually the operating costs are fixed since they are damp leased and engines, block checks etc are all accounted for in the lease rate.
Fuel flow is an issue and the 757 is a good aircraft to substitute on lighter runs which would be not so economical with a 767.
Therefore the 757 and 767 combination is less expensive than a pair of 767, and have more lift than a pair of 757.
The common type rating is helpful but only for pilots and not engineers.