Connect two ECC with one PI

Here is the scenario:

PI1 ECC1 on SLD1

PI2 ECC2 on SLD2

How to connect PI1 to ECC2 for transferring IDOC's to and fro? My thought is that we need to create Business System of ECC2 in sld1 and assign it a logical name. Rest will be ussual.

How to connect PI1 to ECC2 for transferring Proxy to and fro? ECC2 is already connected to PI2 through sproxy and sldapicust.

Your thoughts?