Master Degree Application for LSE Watch
Exceeding the word limit just means you have to edit it so it's within the limit.
So if Economics and Management is your first choice then write a statement for that including management aspects.
But if Economics is your first choice then it should be mainly economics with a little bit about management in case you don't get into Economics but get passed on to E&M.
Having said that, LSE is so competitive that writing a statement to cover two programmes is always going to be difficult - sometimes it's better to choose one and really go for it.