I am not sold on carrying or wearing two shells, although I will keep an open mind.
Up to now, for a local day hike where rain is not expected, I carry a wind shell (GoLite Helios) -- keep out the wind when at rest, or partial protection against a sudden shower. I figure that even in a prolonged rain, it's just a day hike, so no big deal about getting a bit wet (my locale is in temperate southern California).
For multiple-day trips where weather can be more variant and/or don't want to risk getting soaked, I carry a hooded wp/b jacket instead (and maybe pants too). Here, I will use the jacket as both wind and rain shell.
If you are not bushwhacking, you might consider something like the waterproof and very breathable DriDucks (5 oz. for the jacket) for both wind and rain protection. This jacket is no good for bushwhacking, but then, neither is a lightweight wind shirt in all likelihood...