Convert CNC mill to laser engraver

[KPJ-qd]

I've got a CNC mill running through a Delta VFD-M, fed by a USB COM port (COM3), and I've machined a bracket for my laser. LaserGRBL communicates through a different USB COM port (COM8). I've searched everywhere to find an easy way to mirror output to both COM ports, with no luck. I'm running Win 10; it seems like there must be something similar to *nix 'tee' to get the job done, but I'm a command line moron. I figure COM8 won't care about X, Y, and Z, and COM3's motor is unplugged, so there shouldn't be any conflict to mirroring--the Gcode will all go to the right place. Has anyone done this? If so, how?