Only reason I can think of is that there is either other experiments on-board that they need to recover (solar wind collectors, etc), or that they were both taken up in the same launch vehicle, which would be the lion's share of the expense of this experiment.
If you launch the first sat into Orbit A, it still has the orbital velocity of the Earth. Unless you can slow the first sat down a lot, it will continue to track along in about the same place as the Earth.
Waiting 6 months will accomplish very little in that case. The reason is, in 6 months, the Earth and the first sat will be in nearly the same place.
The key concept is known as "delta-v" (http://en.wikipedia.org/wiki/Delta-v). It's the change-in-velocity allowed by a certain amount of fuel. Here's some more on the STEREO mission trajectory: http://stereo.gsfc.nasa.gov/orbit.shtml It's worth it to watch the movie.
Gravity assist can help a lot, but it also takes a lot of time.