Again, it is obvious why you would not want to just open the crossfeed valve, but I don't understand why we are not allowed to actively crossfeed (switching off the pumps on the 'good' tank).
If I zero in on the word, "just", above, can I assume you know that to crossfeed at all you will indeed have to open the crossfeed valve? ... Now, here's the problem... Crossfeeding happens by pressure differential. You're assuming the tank with the leak, with pumps on, will still be providing more pressure in the plumbing than the side with no leak, with it's pumps off. But you have no way to be sure. In fact, if the leak is in some of the plumbing between the xfeed vlv and the pumps, you may well end up siphoning fuel from the non-leaking side, and you won't know it until you watch the quantity dwindle.