Typically, Keystone uses holding tanks of the same size in their trailer models. So, if the "gray capacity" is listed at 78 gallons, it's almost always two 39 gallon tanks. With your black tank capacity listed at 38 gallons, I'd suspect that the actual gray capacity is 76 gallons (two 38 gallon tanks) with some "allowance for plumbing capacity"....
That said,
IF YOUR TRAILER HAS ONE WASTE WATER OUTLET, you can "equalize" your gray tanks by installing a terminal valve on the outlet and then opening your two gray tank valves. The waste water will flow between the two tanks and "average/equalize" between the two. If you do that, remember that you'll have gray water "immediately behind that terminal valve, so if you pull that, you'll get a "greasy shower". That's better than a "stinky shower" from a leaking black tank, but not by much if it's a hot, sultry summer day when the accident happens and you're miles from enough fresh water to wash off the stench on your body and clothing.....
You can order a terminal valve from any RV store, some WalMart RV departments have them and EBay or Amazon have them available. Here's one Amazon link:
https://www.amazon.com/Valterra-T58-...2&sr=8-1-fkmr3