Ways of Connecting 

Back to Back SIMs over UART
You can use both attached projects “A” and “B” with any two SIMs wired back to back with their UART (power, RX<-TX, TX->RX) 

Back to Back SIMs over RS422
For Gen5 SIMs (SIMx52, SIMx62) you can use RS422 (4 wire) by connecting the two grounds and wiring AB to YZ back to back on the two SIMs — make sure you have Full Duplex and Termination selected on both ends.

Single Ended to a Terminal
You can also just use one project “A” or “B” with one SIM wired to an external device via the UART. 

How it works

Both “A” and “B” send and object and also receive an object.

The projects, by default, come up for the SIM231, but you can File->Change Platform/Variant and retarget the project for your specific SIM.