Like everyone has said, yes you can. There's a number of both White Labs and Wyeast "strains" out there are actually multiple strains. Some are regular yeast plus other yeasts (Brett) or bacteria for sours. Other strains are just blends of Saccharomyces strains designed to work together in a certain way. With blended strains from vials/smack packs, the manufacturer has taken care of making the proportions right to the strain ferments like it should, and repitching (or maybe even making a starter) could throw the proportions off and alter the flavor of the final batch, especially with sour blends. Doing it on your own is a little tougher, and unless you know what you're going for it's probably not worth it.
I sometimes use more than one strain in a batch, but it's always with a reason, and usually as a secondary strain. Bugs or wild yeast to sour or add some funk, or a second yeast to dry the beer out.