It's not clear to me that the radar is necessarily a separate constellation. SpaceX hasn't said a whole lot, but for example, SpaceX sells "Starshield" comms to the military, which uses the Starlink satellites for communications, except apparently the military has separate ground stations.
There's also been some published work where people have been talking about using Starlink signals passively for aircraft tracking. I'm not sure you couldn't use Starlink as a "stealth radar", in that the waveforms that are coming out of the satellites look like normal communications, but the satellites can also look at the echoes. Having the entire Starlink constellation form the radar is also pretty attractive from a cost and resiliency perspective since there are so many satellites to shoot down.
There's also been some published work where people have been talking about using Starlink signals passively for aircraft tracking. I'm not sure you couldn't use Starlink as a "stealth radar", in that the waveforms that are coming out of the satellites look like normal communications, but the satellites can also look at the echoes. Having the entire Starlink constellation form the radar is also pretty attractive from a cost and resiliency perspective since there are so many satellites to shoot down.