
I have only had this two litter situation twice, one litter were only 4 days apart the other as with yours about 3 weeks.
With the first two I kept them strictly apart on advice of fellow breeders so that new owne4rs would have no doubts about which litter a pup came from and thereby parentage.
http://barbelka.awardspace.co.uk/wpimages/wp43df22d5_0a_06.jpgWith the bigger age gap again pups were kept strictly apart until there were just two of the older litter left
http://barbelka.awardspace.co.uk/wpimages/wp29172616_0a_06.jpg after the others had gone to their hew homes so they were over 8 weeks, and then the pups were only together under supervision, and there were no issues with the 5 week old younger ones being treated gently by the two older ones,
http://barbelka.awardspace.co.uk/wpimages/wp26833e16_0a_06.jpg who were both gone in another week or so.
So generally for the good of the pups development I'd never mix two litters in the normal course of events, it's not something that would happen in nature, and on the rare occasion two bitches in a pack have pups the younger ones will normally be killed or die..