This may seem a daft question, but why would you need two phones on
different circuits to ring simultaneously? If you need that then put a
double adapter in the socket that rings.
Point of interest, have you set up the ringing definitions correctly?
Otherwise if you need the possibility of being able to answer the call
in more than on place why not put a VoIP app on your mobile and answer
it that way? Either that or get yourself a DECT kit?