Without a full response content it's difficult to provide a valid rexgexp. But from a variable extractor in advanced mode if you use such regexp: "cand":"(\d{1})","intrw":"(\d{2})"\}

with a value template: $1$_$2$   does it work?

Here i assumed that the candidate ID is only 1 digit and 2 digits for the interviewer ID but of course you can customize it to suit your needs.