I posted a whole writeup on how to do that a while back.
You will need to do some slight mods to the firmware (easy) and also to construct a small circuit that kills the AC power to the entire printer at the end of the print.
It also has an added bonus of having overtemp safety switches on the bed plate, assuming you can find a place to mount them on the FT-5 plate (mine has a thick aluminum sheet for a bed).
I have had this installed for a while and absolutely love it.
Here is a copy of the writeup for your convenience-
Automatic power down.pdf (445.3 KB)
However if all you want to do is to shut off the bed heat, you can do that by merely setting the target temperature to zero at the end of a print in the "end of print" gcodes in your slicer settings.
Add a line such as- M140 S0;
The controller will then turn off the drive to the booster and that in turn will shut off the heater.
It is no different in this regard than a stock FT-5 setup.
Oh, and don't forget that with two PSUs running you will need to size your AC fuses for the combined load of both PSUs.
The original FT-5 fuses will probably be inadequate.
If running a full 350 watts to the bed, the total wattage will be on the order of 500-550 watts given that the PSUs are not 100% efficient.
That would be ((bed heater+motors+hot end heater)/0.75) approximately.
So I would suggest using 7.5 ampere fuses to start with.