You should be making starters with your smack packs. Although a pack will ferment out a beer a lot of time, it may stress the yeast out. This means you may have a higher final gravity, excess esters, and possible off flavors.
Buying two packs of the same yeast is a waste of money for most ales as you can just make a starter. Search for Mr.Malty pitching rate calculator on google. You can input your predicted starting gravity and batch size. The calculator will tell you how big of a starter to make. It will usually be between 1-2 liters for most ales.
To make a starter, add about 4oz of dry malt extract for every liter of starter. This should give your starter an OG of about 1.040. (i.e. a 1.5 liter starter will take 6 oz of DME). Boil it on the stove for 15 minutes to sanitize. Cool it to pitching temp and add to a sanitized container with an airlock (1 gallon glass jugs work well). Pitch your yeast and wait 48-72 hours. I usually make my starter 4 days before brew day, let it ferment for 3, and then put it in the fridge the night before. This will cause the yeast to sink to the bottom. You can then decant off the excess liquid, leave a little to swirl the yeast, then pitch. Make sure to take it out of the fridge 1-2 hours before pitching so it has time to acclimate to pitching temps so you don't shock it.
This may sound like a lot if you haven't done it before, but it's a really simple thing to do that will insure you have healthy fermentations. Good luck brewing.