I've used mixed sands many times, works perfectly fine. It's not like using different grit sandpapers... you have how many grains of sand constantly flowing, not glued to paper in a fixed spot.
68 Road Runner, 69 Belvedere, 71 Challenger Vert
340 barracuda, 01 Ram CTD, 95 Ram, 04 Ram, 85 Daytona turbo Z
99 Trans Am, 66 GTO, 06 Magnum RT AWD