connecting buildings

As far as I know, there is no national model code requirement for two buildings to share a common alarm system or that they be interconnected. There may be a local requirement to do so (Denver comes to mind). NFPA 72 minimum requirements are to annunciate by floor and by building. See NFPA 72-2002 4.4.6.1.2 and A.4.4.6.1.2. If your locality prosecutes the IBC, see IBC-2003 907.8.

In my opinion, each building in your example is permitted to be annunciated separately by street address, so long as the building were

I have a nursing home with a connecting walkway and connected the systems together to warn all residents in both buildings to potential danger. if one caught fire it is close enough to singe the other the local ahj agreed it was a good thing to do. however I can not find a code section that requires it.