Double batch...double yeast?

This weekend I will be doing a double boil 10gallons.

Do I need to use double the yeast?
I usually use the smack packs from NB.
(my logic using 1/2 the norm... Yeast multiplies so rapidly does it really matter if I start with 1/2 as much. I am assuming, if you recommend I do this, it will take a little longer to get going and complete)

It will take longer to get going. It is normally recommended to make a starter with liquid yeast, even for a 5 gallon batch. You should be making an even larger starter for a ten gallon batch. Use the Mr. Malty calculator to figure out how much you should be pitching.

Yeah use the calculator... for some styles pitching low to stress the yeast a bit and get extra phenols and esters might be appropriate, but for most beers (especially lagers) it is not. Your starting gravity also has a lot to do with how much yeast you should pitch... When I do 10 gallon ales of 1.050 gravity or less I usually make about a 4 liter starter with one pack/vial or about a 2 liter starter with 2 packs/vials. Put it on the stir plate for a day or two, chill, decant, and pitch the yeast.

Enjoy!