In theory is should be possible to place both game cameras in the same spot (IPD=0), make their frustums such that each render one half of an effective single viewport, glue to two halves together into that single view, and show it identically in both eyes (EDIT: …where “identically” is not to say you wouldn’t crop the appropriate part for each eye, of course :P); And it “should” work with any VR title, without any modifications.
Alternatively: If it is possible to use different resolution per eye (…or even outright request a mono view, through the VR runtime, mooting any trickery), maybe one could minimise the bitmap size of one eye’s view plane (just so that it doesn’t use up many cycles, since you’re just going to discard it, anyway), and render the full mono view with the other.