Using 2 different yeasts...

[first batch paranoia]My first batch seems to no longer have much fermentation activity after only 48 hours.
When I returned home from work yesterday (about 20 hours after pitching the yeast) there was some good activity, but not it seems to be stagnant.
I have the packet of yeast that came with the kit still left (the shop advised me to use a better yeast), should I pitch this second, different yeast in hopes of getting the fermentation going again?
Is this quite period typical?
Should I just wait it out?[/first batch paranoia]

It's probably about finished with active fermentation. I'd let it sit for a week or 10 days and then check the sg. If you're really worried, you could check the sg now and see if it's done. I wouldn't pitch more yeast- you don't need it.

Does the beer have a krausen (foamy head)? What recipe did you brew? What yeast strain are you using and at what temperature are you fermenting? Most likely your beer is fine, if you are worried use a hydrometer to see if the beer is finished fermenting. Some yeast strains can finish quite fast, but there are a number of other variables that can affect fermentation periods, so it can be hard to determine the cause. Pitching more yeast into the beer will not restart fermentation, necessarily. The yeast that is being pitched must be actively metabolizing sugars, so that once you add it to the beer it can continue fermentation.
RDWHAHB!

Thanks for the help, I will just ride it out and see how it turns out!

Checking the specific gravity is the only way to be certain a batch is done.