The problem you'll have is there are no spare outputs to control a second coolant device. If you have a SmoothStepper, you can wire up a second off-the-shelf BOB to give you more outputs, then you can control the extra coolant any way you want. Without that, about your only option is a manual switch to select between the two coolant devices, with the computer only controlling the on/off.
Not sure what the advantage of the proposed setup would be.... If you are using flood for anything, why not use it for everything? Flood works MUCH better than mist or micro-drop, with much less fuss. Mist and micro-drop are useless for deep pockets, or any geometry that blocks the stream. Mist absolutely sucks for drilling.
Regards,
Ray L.