Do a roughing path with the larger bit. Then do a second roughing path with the smaller bit. Set the start height were you left off on the larger bit and the final depth a little lower (otherwise your smaller bit will just move all over the space already cut). I am assuming that is why you are doing 80/20%. I.e. 80% of depth then the last 20%. Maybe you can explain better what it is you are trying to do and why?