empire4,
Not in cruise, but in low level holding, with two at idle and two at whatever was required to maintain the hold speed. The claim was about 8% reduction in holding ff at 1500'. I could never measure the difference when we tried it, but it did work on the P&W JT3D-3B powered B707-320.
In cruise, anywhere near optimum, there is not too much thrust to spare to even consider such a technique.