Didn't have the time to be messing around building H-bridges, so I caved and ordered some easydrivers from AliExpress. 4 bucks free shipping, so not really breaking the bank to get a couple. Got both sleds moving nicely, and wrote a quick python script to direct them by clicking my mouse. rather than flashing grbl I am writing everything from scratch to control the steppers, laser etc through serial - allowing me to do the hard part (g-code interpretation) in python and also opening up other uses without having to re-program the Atmega328.