Sometimes the question asks for 'at least one' related material. in this case, it pretty much means TWO.
In case of this, it is best to know two related materials. Sometimes the question asks for more than one stimulas piece, but very rarely.
Then again, if your related piece takes up more than 2 pages, it may be so in depth that you only need one, even if they ask for 'atleast one'
BUT, if they aks for two, you need to do two, or you risk getting marked down for not answering the question, no matter how in depth your one related piece is.