How to get phones ringing at the same time on two networked NS 700's

[MESH2853]

Hi,
does anybody know how to get phones ringing at both ends of networked NS700's, I have a ICD group on one site taking the main number but I want phones from the other system in the group but it won't let me add them. Do I have to use ps phones and forward them to extns on the other system ?
Thanks

 

[obtsystems]

If you have enhanced licence then you use portable stations fwd to extensions on other system