I have e-filed a return but IRS has not accepted it. It has not been 48 hours yet. The return is due 09-15-2020, tomorrow. Can I mail a paper copy too?

Nothing "happens" to the e-filed return if you mail a return.  But e-filing and mailing the same return can cause confusion and delays if the IRS has both.  You say it has not yet been 48 hours since you e-filed.  Why not give it a little time?  

I will wait until tomorrow but if it is not accepted by 2:00 pm, I'm going to mail it...certified.