Want 2 separate S2 systems

From what source are you playing? You should be able to play different sources on different speakers. Just one caveat here: if you both use the same account for a music service (i.e. Spotify) on both speakers, the first will stop. It used to be Spotify did not notice this, but this seems to have changed with the new app in May last year. If this is the case, splitting into two systems will not help; Spotify will still not allow two streams for one account.

You could however use multiple accounts for the same content service:

https://support.sonos.com/en/article/use-multiple-accounts-for-the-same-content-service

Hello, thank you for your response. I was trying to play BBC Radio and my partner was listening to music on Apple Music. We have a family account for music, so shouldn’t be issues there. The system is likely setup under my Sonos account, but I think my partner has his own, so I’ll check to ensure he is logged in using his account on the Sonos App and not mine. I presume that he can still access our system even if it is registered under my name? If not. would we need to deregister the devices, and register our respective device under our own accounts to rectify this?

Nothing needs to be changed about the Sonos account.  You only need to add each individual music service account using the steps in the link above.  Then you can choose what account to use in the dropdown at the top of that music service’s screen.

I tried to set up another account for Apple Music using my partners Apple Music login but it keeps defaulting to MY Apple login and just prompts for my password. I cannot enter his email and login so it can add it as another music account. 
 

To be clear, 

1. I have Sonos app on my phone and I login with my email to Sonos. I am the ‘owner’ of the system and I have the Apple Music Family plan, of which my partner is a family member (a parent / guardian)
2. If my partner logs into the Sonos app on his phone using his email, he cannot change anything when he uses our system. Message ‘you must log in as system owner’ appears.
3. If I access content services in Sonos app on my phone, and click add account and pick Apple Music, it defaults to my Apple login. 

There seems to be no way to add his Apple Music account as an alternative account for music. 

Apologies if I have missed something along the way, but I am doing what it says in the link. 
 

Thanks

For authentication purposes, adding an Apple account is always going to default to the account of the Apple Music app on that phone.  Have your husband add his Apple account to the Sonos app using hIs phone. 

Hello we have already tried on his phone. That is when he gets the message I mentioned already, saying he needs to login as the system owner.  It seems he does not have permissions to add content services on the ‘system/network’ that the two Sonos One’s are on. 
 

Just keep going round in circles ☹️

Hello, we managed to sort things out earlier. I came here to update and saw your last message. We signed in using my Sonos account on my partners phone, as that seems to be the system owner. 
 

We were then able to add his Apple Music account and on both of our phones we can see both music accounts via the drop-down. 
 

Apologies for the confusion and thank you for your patience. It was hard to understand the need for my partner to be logged in with my email, but on his phone. Not sure if it will still work if he logs in using his Sonos login, but we are leaving things as they are! Don't want to phaff about any more. 
 

Once again thank you everyone for your help 😊