It's totally fine to sign up for both. In the same way, it's totally fine to look for prematch positions at the same time that you're interviewing for match positions.
If you find a spot outside the match, then you need to withdraw from the match.
If you submit a rank list, then you must stop looking for positions outside the match until the match (and SOAP) is complete. This is REALLY important. Some people got in trouble for looking for positions outside the match after not matching, but before SOAP was completed. That's not allowed.
I doubt that any PD will review the find-a-resident site trying to see if you're there looking for spots.