AYO. How do the systems interface? How do you know this? Someone, somewhere, at some point, wrote the interfaces based on somebody's need. Test the known need and whether the provided solution meets the need.
We used to call this "inter-system testing".
Oh, you might want to make sure that each of the systems functions as it's supposed to on its own before trying to plug them together ... unless of course they don't function at all w/o interfacing.