Mostly using 2 packets of yeast in 1 5 or 6 gallon batch does 2 things: increase the initial yeast population, which means the optimal colony population will be reached in about 1/2 the usual time; the effect on fermentation time with this is negligible. The 2nd thing it does is cost more money. You can increase the culture population simply by using a yeast starter, it costs about a quarter to mix one up. I always use a yeast starter.
I've used Red Star Premire Cuvee dry yeast that sat in my fridge for a whole year & had zero problems with it. As always, I used a yeast starter. A yeast starter can save you the trouble of wondering if your yeast is still viable or not; for the cost of a little sugarwater & maybe a pinch of nutrients, you know if it's still good in about 20 minutes or so. Hope you find this info useful, GF.