Each engine in the BE20 is fed from its respective fuel tank by an engine driven low pressure fuel pump. If this pump fails then the engine is fed by the standby electric boost pump. Each standby electric boost pump has a switch to turn it on.
The crossfeed system operates by placing the crossfeed switch to the position of the engine which is to be fed from both tanks. If the crossfeed switch is placed in the left position the crossfeed valve opens and the right electric boost pump activates. However, if this is done with the right engine still operating most if not all of the fuel provided by the right electric boost pump will be sent to the right engine by its engine driven low pressure fuel pump. So no crossfeed happens even though the valve is open because each tank is still feeding its respective engine like before.
But with a max fuel imbalance of 1000 pounds, this is not an issue.
If this is not clear let me know and I will post the system diagram.