https://developer.android.com/reference/android/net/VpnServi...
"There can be only one VPN connection running at the same time. The existing interface is deactivated when a new one is created."
Note this is not about routing some traffic to the VPN and other traffic to the clear net. This is about running two VPN connections simultaneously.
On Linux, I have no problem running either bare wireguard or tailscale alongside Forticlient. On Windows and macOS it's a bit more janky, specifically the DNS resolution, but I don't daily drive these platforms so I may be missing some kind of knowledge to fix this.