For starters, IBActions must have a signature of:
-(void)action;
-(void)actionWithSender:(id)sender;
-(void)actionWithSender:(id)sender event:(UIEvent*)event;
So you can't have an IBAction with multiple arguments if that's what you were looking for.
However, to answer your question, to have a method with multiple arguments, in Objective-C, it would look like this:
- say:(NSString *)textToSay withUserName:(NSString *)userName {
...
}
In AppleScriptObjC, you move all of your Objective-C method parameters to the beginning of the method name, replace the colons with underscores, and put your arguments in the parentheses.
on say_withUserName_(textToSay, userName)
...
end say_withUserName_