Here is a fudge which might work. Completely off the cuff and untested. But if you create a dummy tool (say T3) and assign both bed heaters to it, then you could use G10 to set the standby temperatures for both heaters. You'd likely need to set the active temperatures too. Then if you initial make that tool active, it should start to heat, but when you make the normal hot end tool active, the dummy tool (the bed heaters) should maintain the standby temperatures. To use G10 with multiple heaters, you use the colon separator for the S and R values which means that each heater is given its own set value. So you could run one at say 60 and the other at say 70. Although in practice, thermal conduction might preclude that from happening.
