Like connecting T1 cards directly, back-to-back 56K communication requires a special cable. Also like T1 crossover cables, anyone having bulk networking cable, some RJ45 connectors and a pair of crimpers can make this 56K crossover cable. The specific pinout is shown in the image below (imagine the tab facing away from you).
As seen in the image, a 56K crossover cable requires that pins 1 and 2 are connected to the opposite end on pins 7 and 8 respectively. Unlike the back-to-back serial cables (DTE to DCE), neither end of the 56K crossover cable has any special characteristics. However when the 56K interfaces are configured, one end does need to be configured to supply the clock to the other end.
R1(config)interface serial 0/0 R1(config-if)service-module 56k clock rate 56 R1(config-if)service-module 56k clock source internal R1(config-if)ip address 10.1.1.1 255.255.255.0 R1(config-if)encapsulation ppp R1(config-if)no shut
R2(config)interface serial 0/0 R2(config-if)service-module 56k clock rate 56 R2(config-if)service-module 56k clock source line R2(config-if)ip address 10.1.1.2 255.255.255.0 R2(config-if)encapsulation ppp R2(config-if)no shut
The above configuration, combined with the previously described 56K crossover cable should bring up both interfaces. It is worth noting that the interfaces on either end of the link must be a 56K interface (a T1 card CANNOT be used as a surrogate on one end).
This configuration is a working sample. Like the example I gave with T1 back-to-back connections, it is possible to use these types of interfaces with HDLC and Frame-Relay labs as well. One words of caution is that Cisco has several different models of interface cards. Each model is dependent on certain hardware and/or software combinations that should be researched for compatibility.