I have a MacBook Pro, connected to my private Wi-Fi (Apple Time Capsule) network. I have another Wi-Fi guest network (Devolo). Opening who's on my Wi-Fi shows details of machines connected to my guest network, opening Devolo cockpit allows me to see and configure the Devolo Wi-Fi network, even when I'm connected to the other private network?
Wouldn't the simple explanation be that the networks are routing traffic between them.
Check your router settings, it could be as simple as bridge mode on one of them. Also, map out the DHCP servers and network ranges on paper in case writing it down lets you realize there aren't actually two networks or if one is NAT translated as an address on the other.