You can't borrow from an IRA. What you are asking is, can you take a distribution and return it within 60 days (normal rule) even though you have already taken a $100,000 distribution under the CARES act (which can be returned over up to 3 years.)
The answer is yes. However, remember that on a regular distribution (not covered by the CARES act) there is a mandatory 20% withholding, in this case, $100,000, so you get a check for $400,000. But, you must put the entire $500,000 back within 60 days to count as a reversal of the distribution. You have to come up with that last $100,000 yourself, no matter where from. If not, the $100,000 that you don't put back will be treated as a distribution subject to regular income tax plus a 10% penalty.
You would get the $100,000 withholding back in your tax refund when you file your 2020 tax return to report the 1099-R with the $500,000 withdrawal and then report that you repaid it within 60 days.
