If you’re currently working full time, generally no, you can’t hold a second appointment unless at least one of your positions fits within the exceptions (e.g. summer school teacher in DC, second position pays on a fee basis rather than hourly/daily, etc. See 5 USC § 5533(d-e)). Other exceptions may apply per OPM approval, for quasigovernmental positions not subject to title 5, or as provided separately by statute (e.g. 13 USC § 23(b) authorizing dual employment for Census field work with approval of your regular agency).
You mention “5-day federal job,” but the days don’t necessarily matter except insofar as they contribute to your full-time status (since many alternative work schedules may result in two to four day workweeks, such as on a 9/5/4, 4/10, or Panama schedule). So if you’re currently working five days but fewer than 40 hours per week, you can take a second position for up to the difference without 5533 applying, provided you comply with the relevant regulations of your employing agencies. Keep in mind that 5533 defines the week as Sunday through Saturday for the purposes of hour limits. Some covered organizations, including most nonappropriated fund instrumentalities, use different pay periods and workweeks than appropriated fund agencies generally use, but that doesn’t change how the limitation applies.