Merge walls is a one way function intended to create a profile from a selection of walls and then replace the walls with that profile. The only way to reverse the process is with the standard undo command which of course only works if you've only just done it or you don't mind undoing everything since (and haven't saved in the meantime).
The easiest way to revert these walls would appear to be to change the profiled wall back to the settings of the bottom one and then drag a copy up and adjust its settings to what the upper one used to be. If you still have another with the same settings you can use the eyedropper and syringe (option/alt click and command/ctrl+option?alt click respectively) to quickly match the settings.
Rubia Torres
