Well, you are the first person I’ve ever seen ASK about Bitcoin. 
The problem is you can add an attribute to a form to allow it to accept Bitcoin (accept="bitcoin"
), but there are no attributes in s2Member to stop it from accepting a credit card. I’ve never used Bitcoin so I’m not 100% sure about what a form that accepts Bitcoin looks at (and I don’t have time to test it out for curiousity’s sake). So, I could be wrong and maybe a Bitcoin form doesn’t accept credit cards. If that’s the case, then the answer to your question is in wrapping multiple checkout options into a single form.
So, what I’d suggest you do is create a shortcode with the accept="bitcoin"
attribute at your Bitcoin price and take a look to see if it still allows the use of credit cards. If it does, you still custom coding. If it doesn’t, create a second form without that attribute at your USD price. Then wrap them as shown in the linked KBA.