From what I understand, multiple yeasts are used in a couple of different instances.
The first is when you want some of the flavor profiles from a couple of different types of yeast, so you put them both in to get the combined flavor profile.
The second is if you want to use the first yeast for a certain flavor profile, but it won't ferment as far as you want, so you can throw in the second yeast to dry out the beer a bit more.
I think Muntons is a pretty flavor neutral yeast, so I don't think you'll get a whole lot special out of using it with another yeast, but I really don't have any experience in using multiple yeasts, so I can't really help you out as far as whether or not it's pointless to try.