Guy:
I am very surprised by this. I guess that if the POH for your specific airplane says that, then you should NOT feed both engines from the same tank. However, if you've read the Fuel Management page in my
www.SkymasterUS.com website, which applies mostly to '65-'69 337s, there is no such warning. Why? Because those models -- at least the ones I looked at -- didn't have any such restrictions on their POHs and because I've flown my old '69 337D routinely with both engines feeding off one tank.
Your message prompted to look at the POH for my normally-aspirated 337G with long-range tanks, and I could not find such a warning. Moreover, it seems sensible that you should be able to feed both engines from one tank, to allow you to fly with both engines running if through a fuel leak or for any other reason one tank goes dry in flight.
So, go look at your POH. If it doesn't have the warning, I think you can do it.
Ernie