We have connected Partner systems in the past (before IP Office came out) miles away from each other with multiTech boxes and FXS/FXO to line and station ports of each system via the www.
But in order to transfer a call from site (A) to a specific ext at site (B) the call would have to go to site (B's) AA and vice versa.
Not very pretty or user friendly to say the least. No BLF,centralized VM ect.

So, to answer your question:
Can (2) separate Partner systems be connected to operate as (1) system? The answer would be: No

Avaya SMB Authorized Business Partner. ACIS/APSS
ESI Certified Reseller/Installer
www.regal-comm.com