Running 240v circuit to detached garage
As far as using the existing feeds to power a sub panel, no you can't. You can't parallel cables that small. The good news is, depending on the size of your conduit, you could pull some THHN in it, after removing the 2-12/2 cables. This could feed a sub panel and then you could reroute your two 120 Volt circuits into the sub panel and also add your 240 Volt outlet.