I know the engine computer keeps track of the actual running hrs , but is it possible to adjust the hour meter backwards due to leaving the key on for an extra 50hrs on 1 engine and not the other ?

buy new hourmeters

That happened to my boat for 68 extra hours a couple of months ago when in for service. The gauges cannot be wound back, but you can unplug them until the engine computer catches up.

Yea that's it unplug the other gauge

Dealer did the same dammed thing to my last boat.... bought it brand new and took it back for some minor warranty changes and they left the dammed key on and drained both batteries.... along with black foot prints all over the vinyl..... unhooked the leads to the hour meter and logged hours manually.. then plugged it back in....

Leave the key on for another 950hrs

While my gauges were at Livorsi for updating, they asked if I wanted them reset to zero. So I assume they could reset them to any number you wanted. Be easiest justto unplug the high one until the lower one catches up.