doubling up on my yeast

should i do this. i notice lots of online videos where people use two packs if they don't make a starter.

pro/cons?

Sure, you can do that, if two packs is the appropriate amount for your batch.

Check the pitching calculator at http://mrmalty.com .

There is an online calculator that will tell you how much yeast you need to pitch based on certain parameters:
Mr Malty Pitching Rate Calculator
Edit: I see Rick500 beat me to it.

For a 5 gallon batch of 1.054 beer, 2 packages is the proper amount to pitch (assuming near 100% viability). For higher gravity beers, you would be under pitching, and for lower gravity beers, you would be over pitching.

If you under pitch, your fermentation could be slow to take off. This leaves the wort vulnerable for an infection to take over (although highly unlikely). You will also stress the yeast and they may produce some undesirable characteristics in the beer.

If you over pitch, your risk of infection goes down, you don't stress the yeast, but your fermentation may be too fast and your beer may lack character.

I'm sure there's more, but that's all I can think of at the moment.

If you have to choose a side to err on, Dave Logsdon (owner of Wyeast) recommends under pitching based on feedback from professional brewers.

thanks guys