In my case, my nvr is here at home and the other nvr is at my in-laws house. There two (Hiks) out of the three cameras are connected via poe into the nvr. The third is a Dahua camera that is powered via a poe injector and then is connected into the router.
When I went to add their NVR into my nvr, I didn't anything, no way to select which cameras I wanted to see off of their nvr. So mine are on different locations and it still didn't work.
lowpro: I would presume that you would need to forward the relevant ports for this if both locations are behind a NAT?