I have never heard that you should not feed both engines from the same tank, and I thought I knew a bit about the subject -- see the Fuel Supply Management Page in my "backup" site at
www.SkymasterUS.com
It's not recommended that you take-off or land in that configuration, for obvious reasons, but why would the option of cross-feeding an engine be included if it should not be used?
Because most of my flying is over water, I routinely excercise the cross-feed function and operate at cruise with both engines feeding from the same tank, and it's never been a problem. In fact, I was once coming back from a lengthy Search and Rescue mission on my '69 with four separate tanks and, for reasons I can't recall, I was running both engines out of the same tank when that tank ran out of fuel. Behind me were two passengers who had begged me and my co-pilot to search for their family members lost at sea. You should have seen their faces when both engines quit. I knew where my fuel was and got both engines restarted in seconds.
And on your first subject, an engine stopping, I would certainly consider as one of my options re-starting the engine with fuel from the other tank. I would excercise caution by making sure I understand the problem before I try the opposite-tank restart. For instance, if the engine died from lack of fuel because of a leak in the tank, then (as explained in my earlier message) I would continue with a single engine.
Ernie