From your description you will need to set up routing between the two sites. Specifically try doing the following:
1) Allocate a new Layer 3 IP Subnet for the Comcast L2 Ethernet Private Line. For illustration purposes in this example, lets call it 10.255.255.0/24.
2) At site 1, put the IP address 10.255.255.1 on the port that connects to the Comcast CPE Box (E.g. Connection to the Comcast Layer 2 Ethernet Privae Line Service).
3) At site 2, put the IP address 10.255.255.1 on the port that connects to the Comcast CPE Box (E.g. Connection to the Comcast Layer 2 Ethernet Privae Line Service).
4) Next you will need define static routes to point to the subnets at the other site.
5) So for site 1, add a static route for each subnet at site 2 with a next hop of 10.255.255.1
6) Similarly for site 2, add a static route for each subnet at site 1 with a next hop of 10.255.255.2
With the above you should now be able to route between the sites and all subnets should be reachable from either location.
--Jon
P.S. Rather than putting an IP Address directly on the physical interfaces that connect to Comcast you could create a L2 VLAN for it and then assign the IP addresses to VE"s associated with that L2 VLAN. This would give you more flexibility in the future if you want to define more than one VLAN going across the Comcast L2 Private Ethernet for different purposes.