There is also DINSTAR as gateway.
It is not clear what the original purpose of the 2nd UCM was. Perhaps it was meant to be another e1 gateway, maybe not,
To me, the 6510 is a relatively powerful appliance based PBX. A T1/E1 connection seems limited in its ability to meeting the capability of the 6510 in regards to its call capacity. Not knowing the need of the client makes anything I post nothing but conjecture, but you do have the options of -
1, Using the 2nd UCM as the T1/E1 interface and peering between the two.
2. Simply going with SIP provided you have decent Internet and a provider who can meet your expectations.,
3. Getting a T1/E1 gateway and:
a) Get a two port gateway and let this device accommodate both lines.
b) Get a single port to augment the existing port on the UCM you already have
4. Get a HA100 device and then use both UCMs. The first one as primary and the other as a failover secondary. You can probably do this scenario in both 2 & 3 above.
Personally, I like #2 combined with #4. SIP is less costly and more flexible and with the HA provides some level of redundancy. However E1, while more costly on a per line basis is generally considered to be more reliable, but then it really depends on how delivered.
In any event, good luck.