round trip delay between the servers should be 30ms. that's what is supported.
Normally the media pass through one of the server and not between the servers. Apart from the media lot of signaling pass between the two servers. Also its to be noted that we have seen cases where customer has MTU size limitation on the WAN routers causing calls to drops.
For communication the two VCS servers created a tunnel so it would be good to take a look at cluster creation deployment guide and to check what ports need to be open just in case if you are incorporating the firewall between the two sites.