It’s because by default, they both use the same port (can’t remember what it is off the top of my head). You should configure one of them to use a different port; preferably WebUI because it’s easier to configure the port for that one.
Although I am curious as to why do you want to use SSP? SSP is a bit troublesome to setup and is in low support. If the idea is to manage software and let the end user decide when to install it, why not use the traditional BigFix ClientUI offers or the Self-Service Application (SSA) instead? SSA is the current preferred method as it has a more modern UI and is currently in active support.