You can have up to 32 controllers running your system.  So all phones, iPhones and Android and PC/MAC controllers can all work as one.

Think of these all as remote controls, the work is done on the speakers not the controllers so any amount of controllers (up to the 32) can be running and setting up music simultaneously.

It’s one of the key features of the Sonos system.

I better not attempt this as now the App on my second IPhone shows ‘Sonos system and mobile device on different networks’ which they aren’t. 

That certainly suggests the two phones are connected to two different WiFi/subnets. Normally speaking, ​@bockersjv  is correct, when all controllers are both for the same version of Sonos (S1 or S2) and on the same WiFi / subnet. 

Only one wifi modem in house so it’s not that also noted that the Sonos community cannot be login on second IPhone as well. Must be technical glitches with Sonos which doesn’t surprise me as it’s very clunky to use. Sound is good I think I’ll leave it all on one IPhone. Thanks for advice 

Leaving it on one iPhone works. Your other iPhone may still not be on the same (or any)  WiFi or subnet, but as long as one is working, you’re fine.

I use two Macs, two iPads, and an iPhone to control one Sonos system. 

Folks end up with many paths to the internet, wifi, mobile data and VPNs are common. Try disabling all but wifi, make sure it is connecting to the same wifi network as the Sonos, and things should work.

Thanks everyone for your input today I discovered the blocker in case others have the same issue. Go to your Apps on the Apple IPhone select Apps > Sonos > Local Network then set the slider to the right for the green icon and hey presto second IPhone now connects to Sonos.