This is a script to help implement this scenario : http://technet.microsoft.com/en-us/library/bb676395(v=exchg.141).aspx

$domain = "mydomain.com"
$remoteHUB = "10.10.10.10", "10.10.10.11"

# Create Receive Connector

Get-TransportServer | New-ReceiveConnector -name "Relay $domain" -Bindings "0.0.0.0:25" -RemoteIPRanges $remoteHUB -Custom -PermissionGroups AnonymousUsers 

# Create SendConnector

New-SendConnector -Name "Relay $domain" -AddressSpaces "smtp:$domain;1"  -Enabled  -SmartHosts $remoteHUB -Usage Custom -whatif

#check with telnet on port 25 that each HUB can access the remote HUB
Read-Host "Check that telnet on port 25 work for each HUB -> Remote HUB"

# Create the domain as Internal Relay
New-AcceptedDomain -name $domain -DomainName $domain -DomainType InternalRelay