The only way that I see -700 FO's being displaced to the -200 in IAD ( a double displacement if you will) would be if junior captains bid FO in order to stay in ATL. Somebody correct me if I'm wrong, but according to section 24 G(a) of the CBA, which lists the order in which people get displaced, the most junior pilot in the same CATEGORY in the system is the first one displaced and then continues until that condition is no longer met. At that point, then (this is a key word in this section) it goes to 24 G(a)(2) and so on. The definition of "Category" is in section 2 and is a combination of pilot status and aircraft type (aka FO CR2). In other words, -200 FO's that are senior to -700 FO's will be displaced before their junior counterparts on the -700. This bid also does not include a -700 vacancy or reduction so -200 FO's will not be able to bid over to avoid it. While that might not be fair, unfortunately, that's the way it looks like it will happen.

Of course, this is all assuming that not enough people voluntarily want to go to IAD, so it might be a moot point.