Okay, so it sounds like WSDL1.1 specification supported the use of overloading. In order to do this, you need to provide different arguments to the input signature. I don't know if you can simply modify an action (it doesn't sound like it I'm afraid :/). So in otherwords, I don't believe there's any way to perform the task you are looking to do (1.2 is the WSDL version).
I don't think there's any way to trick it directly. I'll try a few things in both 1.1 and 1.2 to see if it gets me anywhere though (PHP doesn't cope well with overloading in any situation). Otherwise, methinks you'll need to create unique names for the soap operations (and use your backend to map whatever you need from there).
header('HTTP/1.1 420 Enhance Your Calm');