HDLC is an encapsulation, not an interface type. Because you mention coax, you're probably dealing with T3 interfaces.
Connect the TX of one side to the RX of the other side and vice-versa. Configure one of the devices as clock source internal and the other as clock source line received from the interface. It doesn't matter which is which. Assign IPs within the same subnet to the interfaces and appropriate static routes or configure dynamic routing protocols to use the link as appropriate.
If there's an option for line build-out (LBO) or cable length, set it to 0 or short length cable.
The cable is a 75-ohm co-ax with BNC type connectors. Search for "Dual 735 cable" if you don't have it on hand.