Routing between two networks

Dear Sir,
At my office I have two networks
Network A: 172.18.0.0/255.255.0.0
Network B: 172.20.0.0/255.255.0.0
Both network runs indepedently with it’s own diff. ADSL ISP. and no probelm.

But now We want to connect these two networks ( Network A and Network B )
so that Network A can access Network B and Network B can access Network A.I thinks some sort of routings needs to be done for the same but I don’t know how i.e whether I need two router or only one router also what routes must be added to which router.
I would be gratefull If you can give me suggestion on this.