It depends on what you're going for, but you could easily make a great beer using only one malt. The quick run down on the crystal malts is that the lighter (i.e. 10L-40L) one have more honey flavors while the darker ones (i.e. 80L-120L) have more toffee and raisin flavors. As for whether there's a point to adding in BOTH 20L and 40L, I guess there's only a point if you see one in the beer you want to create. As a style, pale ales vary from being drier and having a more biscuity malt flavor to being sweeter and more carmelish until it approaches an amber ale. 40L would give more carmel and if I were to use the 20L for a lighter, more biscuity version I'd probably just use victory malt instead. Jeff Zainasheff gives an american pale ale recipe like this that uses .5 lbs of wheat DME and .75 lbs victory malt that looks pretty damn good. Obviously, the choice is yours. Hope that helps. If not, I'm sure you've run across this:
http://www.homebrewtalk.com/f12/ingredient-guides-25383/