The short answer is that, yes, it's ok to use that yeast again. The longer answer involves telling you that you don't want to put fresh, new wort onto a massive collection of trub, old yeast, dead yeast, good yeast, and mutated yeast. This can do some really wonky things to your flavor, particularly in a beer without really, really strong flavors. For, say, an Imperial Stout, you could get away with it (but I wouldn't do it anyway).
Use sanitary practices!
Leave a bit of beer remaining from the first batch on the top of the cake. Swirl it around until the cake breaks up. Pour the mixture into several see-through, sanitized containers that are tall and fairly narrow. Glass mason jars work.
To make this easier to see, boil up some water, cool it off to room temperature (be sure it's cooled to room temps), and pour it over the yeast in each jar. You want the bottom quarter of the jar to have the yeast mixture and the top three-quarters to be liquid (this can be beer or the sanitary water I just mentioned).
Cover the jars, give them a good shake, and let them sit for a while. You'll see the mixture begin to form sediment layers, the bottom of which is the trub, dead yeast, and older slow yeast. You don't want this stuff. The second, creamier layer is the good, healthy yeast. These guys you want! Eventually, you'll start seeing little particles float and settle atop this creamy layer. This is your protein materials and mutant yeasts. You don't want this stuff either, so just pour off all of the liquid on the top (including those particles I referred to), then pour the creamy good stuff into another sanitized container. Discard the remainder. To make this easier to see, boil up some water, cool it off to room temperature (be sure it's cooled to room temps), and pour it over the yeast in each jar.
Repeat that with each of the jars you filled and you'll be in good shape to ferment the next batch with only the healthiest, meanest, and leanest yeast in your colony. These guys will get the job done faster.
If there doesn't seem to be enough there of the good guys, you could make a starter with the ones you're able to collect.
