Yes, this is one way you can collect two federal retirements. The other would be toyear) twenty in the military then do a GS career, starting from fresh and not buying in to the FERS. If you buy in using your active duty time, and you are past twenty years, you have to give up the pay but not the benefits (tricare, commissary, etc).
The downside is that neither the reserve retirement or FERS retirement checks start coming until you’re close to sixty, whereas after a full twenty years in the military you start collecting immediately. The other downside is that in the reserves/guard, you can get your twenty year letter after doing twenty “good years” (met drill requirements or were on active duty long enough), but they literally count the days and divide by 360 to reach how many “years” you have. So for me, I’m getting out around 15 years. If I do five years in the reserve, only drilling, no more active duty time or deployments, I’ll do around 40 days a year. That’s 200ish days, not even a full year. So if I stay O5, I’ll retire as an O5 with 15-16 years, so 40% of monthly pay (16x2.5%) and won’t get that money until I’m 60. You can add days (“points”) by doing other things like online training, PHAs annually, etc.