Two networks - one Controller ?

It autoswitches between systems in two locations, i.e. when it can only detect one at a time. Is that true in your case?

No. When I switch to the second network, the controller discovers the sonos device BUT the only options available are Exit or Reset Controller.

This is the windows PC controller

I don't think autoswitching applies to your situation then.

The Desktop Controller is capable of associating with multiple systems ('households'), on different networks. Just use the 'connect to existing system' option. You may have to dig around and perhaps find 'Add another system' or some such.

The current system can be disassociated using the 'Forget Current Sonos System' option.

Okay - so I set up the first association with 'Connect to existing system'. Switched network and did the same. I'll try the Add another system option.

Still getting separate 'Reset' when switching .... Baffled.

Let's take a step back. Why do you have two separate networks, each with its own Sonos system? Or is there a different setup?

The TV is in the lounge - Beam attached - one dedicated router 1. The other router 2 runs all the other devices (smart speakers, Play 1s.)

When I installed the Beam it wouldn't connect to router 2. I'm reluctant to reset the Beam as it was a bugger to set up with Alexa!

That's not a "why", that's a "what". John B asked "why", as in why would you set your network up like that? You could easily set up the second router as an access point and have everything on the same network.

Two separate ISPs - one is adsl. the other is wifi. 30m separation between the routers. 6mB on one 20mB on other.

I live in an isolated location - if one ISP goes down, the other still works.

Nevertheless, the only way to 'share' the controller seems to be to reset it each time