Autos will do ok under 12/12 light but there WILL be some drawbacks... there will be more stretching, the grow period will be longer and you'll probably get a lower yield... but there's nothing that says this CAN'T be done... after all, some folks grow their autos outside and those don't get 18/6.... So for a little while, as you flip your photos, they'll be ok...
Good luck!