It looks like you have an ISR that detects values from the HC12
I presume you need a second similar ISR to detect values from the 315MHz device
I would take the code that saves to the SDCard and put it in its own function. That would make it easy to use with data received from either device.
While you say "without manual switches, receiveng both frequences at once" I don't understand the circumstances in which you would be trying to receive from two transmitters at the same time.
If you need more help perhaps you can provide a description of what each your functions does. It is too time consuming to try to figure that out from the code.
What is this thing grabbing codes from? Is it legal?
...R