Integrating two PI systems

Hello,

I know it is possible to connect two PI systems on different Landscapes to communicate via XI adapter. What if I need to send IDOC from say frp,

ECC1 --> PI1 ---> PI2----> ECC2

What kind of setup would be required ?

Also to connect two PI systems what kind of configuratio is required ? SAP Help does not go in detail regarding the connection set up between two PI systems.

Is using MQ series between two PI systems a better option instead of connecting two PI systems directly ?

Thanks,

Teresa