Bandwidth is going to be an issue with any solution where you will be viewing cameras at a offsite location. This is because you will have to send those camera feeds over the internet (through a secure VPN) in order to view them. If bandwidth is really going to be an issue, then onsite viewing is the only solution which by definition means you won't be able to see the cameras located at the second site. However if your two sites have unmetered internet connections, then there are several solutions I can think of.
First, it should be possible to watch/record all cameras using a single BI interface. Of course this means setting up a full time VPN tunnel correctly between the two sites. I would still recommend that you set up a BI server at each site to record and monitor the local cameras. However, because the VPN allows you to see the individual devices across the network it would be trivial to add some/all of the offsite cameras in BI. This way each server would see all the local cameras as well as any desired offsite cameras. Of course the two systems would be duplicating some of the cameras and you would be reliant on the internet service to see any remote cameras. This is why I would still recommend having two independent BI service - on at each location. That way if the VPN or internet went down, each local server would still capture all their local footage.
Another solution would be similar. Any CCTV camera app (like Tinycam for Android) that pulls RTSP feeds from cameras and displays them in a GUI would be able to show local and remote cameras as long as there is a VPN tunnel connecting the two sites. So you could easily create an interface that showed cameras from both locations.
Also, the BI Web UI3 interface would be available too. For example, I have a full time VPN tunnel that links my home network to my parent's home network (we use it for automatic offsite backups). I can pull up my home's BI web interface at their home the same way I would if I was sitting at my home. Of course that wouldn't combine two unique servers into one web GUI, but you could easily have two web pages open with one for each site.