How to sync 2 iPhones on Roon

Unable to sort out syncing 2 iPhones on roon to independently control the volume. Please give me advice.

Can you please clarify your question?

In case you are controlling the volume of a Roon server, either iphone is able to do that independent of the other.

Or do you use your iphones as endpoints?

If I am listening to room my wife’s iPhone does not show the program being played on our hifi system and therefore cannot control the volume of the hifi system

worked in the past but no longer does

Are both iphones running the latest version of Roon?
Did you try to to install Roon on your wife’s phone?

reinstalled soon on 2nd iPhone and now working. Thanks