OK you have a lot of confusing information.
Are office 1 and office 2 in 2 completely different locations? Or are they in same building?
If same building, are they plugging into the same NTU (same ISP device) for internet?
If they are on separate NTU's - or in different locations, then it is entirely likely they are not part of the same broadcast domain, and your ISP would need to bridge them together in some way.
You'll need to provide more information on the hardware physical topology so we can give you a more solid answer.
In general to your question though, if you add an IP address on the SFP port on both ends, it should not affect anything to do with your internet - as the ppp connection will be providing the internet IP, default route, and your NAT statements should be using the ppp interface. So as a simple test, you should be save to add (as another poster suggested) 10.0.0.1/30 on one, 10.0.0.2/30 on the other, and see if they can ping each other.