You cant. It complains that some other VPN is already running.

There's a somewhat complex way to use it together with another VPN app, with work profiles, see see https://itsignacioportal.github.io/netguard-pdnsf-any-vpn-co...

But in case the VPN app supports running as a simple proxy, without using the VPN service, you can avoid work profiles and just have NetGuard connect to it.