Sounds like a situation whereby the current S2 system was created (as an upgrade) from an existing S1 HH and that both therefore have the same HouseholdID - so both systems are operating in what is usually referred to as ‘mixed mode’, rather than two entirely separate S1/S2 systems.
Usually, after upgrading, the S1 devices to S2, the S1 devices that are left behind should ideally be factory reset, as should the S1 App too.. and from that a brand new S1 HH ideally should be created, which will then have an entirely separate Sonos HouseholdID to the S2 system.. and when doing that, (as the saying goes).. "Never the Twain shall meet”. See this support link:
https://support.sonos.com/s/article/4845
But, there are issues with this, as the setup for a new S1 Household is now a slightly more difficult process these days, particularly if the S1 Household still has S2 compatible devices and the S2 App is also installed on the mobile controller that’s to be used for the S1 setup - but those issues are covered in other posts/threads on the forum. In summary, it’s best to switch off the S2 HH devices and remove the S2 App before setting up the S1 HH, which can only be done with an S1 (only) compatible product, such as a Play:5 (gen 1), just to mention one example. The S1 (only) compatible devices are mentioned in this Sonos support link: (see ‘S1 Required’ products list):
https://support.sonos.com/s/article/4786