RD does not allow multiple Gun Times - the Gun Time is considered to be whatever is the first start of the day. All other times are considered a Wave Offset from this initial start. We allow Wave Offsets to be defined by Division, Age Group, or Participant. You will want to use Wave Offset by Division so that anyone who is entered in your 5k Division will automatically have your Wave Offset Time subtracted from their Finish Time.
You can set this up under Tailor > Options > Finish Line > Wave Start.
You'll now see this new field in Tailor > Divisions:
You'll want to set this to whatever the elapsed time is from your Gun Time. So if your 10k starts at 06:00:00.0 AM, and your 5k starts at 06:30:00.0 AM your Wave Offset for the 5k will be 0:30:00.0, and your Wave Offset for the 10k will be 0:00:00.0 (using the original Gun Time)