This question already has an answer here:
I am playing with a friend and we started cities in the same region but quickly found that we really couldn't interact with each other because they weren't connected by road or rail. Services couldn't be volunteered. Money couldn't be gifted. Workers and students wouldn't commute.
We did notice that upgrades to the City Hall would affect the whole region however.
Besides this, are there any other bonuses one city can offer another within a region if they aren't connected? Is it possible to connect these cities somehow?