That might work, however, you would have to determine what the voltage level would be on an open input to the spindle and laser controller.
For example, if the laser input is pulled high, then when you moved the switch to open the input on the laser then the laser would come on full power. Same thing for the spindle.
If the inputs to the controllers are pulled low then the disconnected device would be off.
If inputs are floating, it's anybody's guess as to what you would get.
If you put that same three position switch on the power wire (assuming both used the same voltage) to the spindle and laser then it would work as you want it to.