# 6 white and # 8 ground in separate conduit back to panel will probably be cheaper than re-pulling it all. i'd also check with a local electrical supply house on the wire pricing. The big box stores aren't always the cheapest.
Sorry, but you definitely don't want to do that. It is a code violation for a very good reason. If they were in metallic conduit run side by side, you could get inductive heating in the conduit. All the conductors for a circuit must be in the same conduit.