Update 5/31/2012: I haven't exhausted this new source yet - I'm back another generation to a set of 4th great grandparents!
Update 6/2/2012: More twins - my 2nd great grandmother (the twin) married a man who's father was a twin! From my Swedish great grandmother, I have found my 2x great grandparents, both sets of 3x great grandparents, 3 of the 4 sets of 4th great grandparents, and one set of 5th great grandparents. A week ago I only had names and birth dates of the 2x great grandparents - much progress has been made!