You should be able to do this, but you wouldn't use the subject line a/b test, you would use the "Whole Email" test instead: https://community.marketo.com/MarketoArticle?id=kA050000000LC9XCAW
The reason for this is because you
might be able to use tokens in the subject line test, but you wouldn't also be able to line up the copy of the email with it (i.e. switching out token 1 for token 2).
If you create two completely unique emails with their respective tokens, and then test those, it should work.