Thanks for the replies, guys.
On the Embraer, one line in the 'one engine inop approach and landing' QRH, is fuel crossfeed OFF.
I believe this is legislative In basis, and not for any technical reason, but it's still in there.
Clearly, we couldn't reset it to OFF as that would result in an engine shutting down while on final approach.
In our sim scenario, the contents of one tank simply reduced rapidly to zero- a big leak- so all the fuel on one side was quickly lost, and the other side was unaffected.
I maintain that the purpose of the crossfeed is so you can run both engines from one side, so why not do so?