How would an attacker know the Gateway IP's of 2 VPN endpoints?

My office has two Sonicwall Routers each at a different physical location and with two different ISPs.

There is an SSL-VPN tunnel between the two.

Today I noticed that the same Chinese IP address attempted to connect to the External IP's of both of these devices on coming from the same port.

Source: 61.160.213.108 port 10000 Destination: Each External IP of our SonicWalls and different ports

Any idea's as to how they could know there was relation between the two routers?