In site a add x.x.x.255 (where x.x.x is site b's range)

In site b add x.x.x.255 (where x.x.x is site a's range)

Eg

Site a ip address is 192.168.1.5
Site b ip address is 10.0.0.5

Site a add 10.0.0.255
Site b add 192.168.1.255