How practical might it be to run a driveshaft between the two props, just ahead of the engines, and set up a gearbox/clutch arrangement, so either engine can be shut down and the other one drives both propellers?
It would cost weight, add to the price, and be another point of possible failure. But the idea is, to increase range and endurance by saving fuel for long somewhat slow cruises.
Would the added weight impair performance, or cut into fuel so much it negates the advantage?
Some time back some other P-38 thread introduced me to a long thread somewhere else arguing that the USAAF didn't have to wait for the Mustang to enable long-range escorts for bomber strikes, the P-38 was already good enough to do the job, it's just that Army politics didn't want to use it for that purpose. Increasing range further, if it doesn't impair combat performance significantly, might have further encouraged such uses if the attitude were different.
And of course in the Pacific war, range and endurance were golden things.