If you dry hop during fermentation, the large amount of co2 that is being produced will drive those volatile aroma esters right out your airlock, instead of in the beer where you want it. So it sort of defeats the purpose of dry hopping for aroma anyway.
Without going into the whole sorted tale which you can look around a read for yourself, you really shouldn't rack to secondary until fermentation is complete anyway.
And nowadays as more and more brewers are opting for long primary/no secondary (again you can look around for the discussions) many folks are dry hopping in the primary, after fermentation is finished. My last two batches I dryhopped in primary on the 14th day for another two weeks. They came out great.
When I used to secondary I added hops on the 14rh day after yeast pitch for 2 weeks. Or on the 21st day for 1 week.