linux networking – network-manager won’t leave two networks active

I have (virtual) Debian desktop, with two nics, connected to two networks, each with a working dhcp and default gateway.

I want to be connected to both networks, but when I turn one network on in network-manager gui, the other turns off, and the other way around. I can not have two networks enabled at the same time.

I have set one of the nic’s to “use only this connection for ressources on its network”; I presume this is to keep the default gateway on the other nic. But this did no change; I can only have one network active at the same time.

Should I just try going without network manager and set up the network “the old way” in /etc/network/interfaces?