Hi Peter,
M13,M14, and M15 are not defined in KMotionCNC. But you could configure M103, M104, and M105 if you wish.
I think the overall most general approach is to treat dual/triple Spindles as if you had a Tool Changer and as if you were loading different Tools. The "tool change" program in KFLOP can then set a global state (Virtual Bit or persist variable) in KFLOP to direct subsequent M3,M4,M5, and S actions to the appropriate device. The Tool Table length and xy offsets can then be used to compensate for the second spindle. There is also a ToolTableSet.c example that you might use to "touch off" each spindle and set the Tool Length for the current Spindle.
HTH
Regards
TK