You can take a second job while on furlough and receive 80% pay from your original job and 100% pay from from your new one. The only proviso is that you must remain available to return to your first job if the employer decides to stop furloughing you and/or you must be able to undertake any training activities that your first employer expected you to carry out during the furlough period.
Just as with anyone who takes on a second PAYE job, while still employed in a first one, you won't be able to present your new employer with a P45, so you have to complete the 'starter checklist' for your new employer instead. That will ensure that the PAYE system looks at your
total pay and deducts the correct amount of tax. i.e. assuming that your personal tax-free allowance has been swallowed up by your first job, you'll pay 20% tax on every penny you earn from the second one (unless, of course, your total income places you in a higher tax bracket).
There is no 'claw back' within the Job Retention Scheme under such circumstances. You can keep all of the pay you receive from both jobs.
https://www.gov.uk/guidance/check-if-you-could-be-covered-by-the-coronavirus-job-retention-scheme#while-youre-on-furlough