Let's say the extensions on your 61C are all in the 6100's and your DID block is 987-654-6100 to 6199. Also, let's say the extensions on your 11C are in the 1100's and the DID block is 210-543-1100 to 1199.
On the 61C, you'd build a DMI that inserts 1210543 and an RLI that "points" to that DMI- both from LD 86. In LD 87, you'd built DSC 11 that "points" to the new RLI. This will give you 4-digit dialing from the 61C to the 11C.
Then, in the 11C, you'd build a DMI that inserts 1987654 and an RLI that "points" to this DMI. Then build DSC 61 that "points" to this RLI and you'll have 4-digit dialing from the 11C to the 61C.
Note that my example assumes long-distance dialing between the two sites. You won't need the leading '1' in the DMIs if it's a local call between the two cities.