To answer your question, no, there isn't currently a way to run two tests.
What you're describing though, is essentially a whole email test. With a whole email test you can change anything about the two emails, so you could have any subject line you like in either one, and make the bodies look as similar or different at you choose.
Yeah, what is the use case? You can test everything with a whole email test.
There are couple of ways to run a test in Marketo. If you can share the complete details on the type of test you want to run, we can recommend the best way.