Yes, there are several ways to accomplish what you are trying to do, and some of those ways don’t involve the config at all. You could just have the wires run from one of the big MOSFETs to a plug (or terminal block) external to the Smoothie and plug in either bed. Or you could simply change the pin assignment in the config from MOSFET 1 (e.g. pin 2.5) to MOSFET 2 (e.g pin 2.7), then you would be controlling the second bed (again assuming you have enough big MOSFETs on the Smoothie because if you are running at 12V you cannot plug a 40W hotend into a small MOSFET because it will pull too much current.).
Or you could get more complicated and connect each to a big MOSFET and create a new module and name it bed2 (or whatever), and have your program access that bed instead of the first one. Or you if you have only 2 big MOSFETs and one is being used up by the hotend and the other by bed1, you could use an external MOSFET and control that via a Smoothie small MOSFET and externally control the bed. Many ways.
Before you stack two heater elements together, then try and run a 12V element with a 24V PS (not a great idea even with PMW) and use an external MOSFET, I encourage you to think more about the problem. What you are proposing is a very inelegant solution to the problem of “I want to save a few bucks on my electric bill”. I’m not trying to be argumentative with you, I’m just trying to get you to think about it, because your power consumption cannot be that high—your stock PS is only 360W. My advice is to insulate the bed and that will help a lot and don’t not burn down your house trying to literally save a few cents.