I think you need not configure two UARTs for this case. This is a SIM808 issue where the device has configured one UART line to transfer both the GPS and GSM data.
So, we could look at it as a simple UART communication between PSoC 5LP and SIM808 irrespective of what SIM808 functionalities are. I don't think the communication is as simple as sending AT commands either.
Here is the link where you can download the APIs for SIM808 - GitHub - DFRobot/DFRobot_SIM808: SIM808. There is a catch here. You have to convert the cpp files into .c files before using in the PSoC5LP project.