Just do it. Mark the amended box. Explain why you are amending.

I'm guessing a 2nd amended return can't be efiled - but I've never had to try it.

Tip #2:

Make sure it is the final amendment.

Agree.  Triple, quadruple, quintuple, sextuple, septuple, octuple, nonuple, dectuple check the return to make sure it is perfect.  All these amendments could be an invitation for the IRS to sniff at the return a bit closer.