In the case of a failure of engine 1 (which seems a bit more likely since this engine is fed by gravity only upon reaching FL150), engine 2 would still be running on its RH tank with the operational fuel pumps. Here one could indeed consider the opening of the X-feed valve and sideslip in order to get the fuel from the LH side into the RH side as this scenario would be fuel critical and an imbalance would evolve quickly...
You can't transfer fuel from tank to tank, you can only feed the tank fuel to the engines. In the scenario above, if you anticipate that you will loose No2 engine due lack of fuel in RHS tank yet have plenty of fuel remaining in the LHS tank, then it would be wise to turn off all fuel pumps, open the X Feed and gravity feed both engines. By Side slipping, the higher tank will preferentially gravity feed both engines.