doubling yeast wont get you more attenuation. I've made roughly 10 all extract batches, never did i once get it past 80% attenuation. I was always matching what mr malty recommended to pitch while keeping ferm temps in the low to mid 60's (first 2 at 70 and same results). It didnt matter if i was using briess or muntons... The highest i ever got was 79.89% (6 weeks ago!). I've tried double pitching, i tried 1.5 pitching... same thing every time. No way you're getting that low of a fg.