I guess it would depend on how flocculant the yeast I'm using is. The general idea would be to maximize whatever benefit the yeast/hop combination provides with the first dry hop (hence to make sure that most of the yeast is still active and in suspension), and rely on a massive second dry hop to make up for whatever I lose. Here's a quote from tnlandsailor based on one of Matt's talks that he attended (http://www.homebrewtalk.com/f13/dry-hop-ipa-64751/index2.html#post668593):
Once the beer starts fermenting in the primary, let it reach high krausen and then start to crash (about 1 day or so after high krausen). Fermentation is still going on, but at a lower activity level. Add your dry hops to the primary at this point and let the beer continue to ferment with the dry hops for 3 days total. After 3 days, fermentation should be complete and the beer should be racked to a secondary.