I agree with Jonathon - practice good sanitation and follow a yeast calculator for pitch rate. The most I ever repitched a yeast for lagers was somewhere in the mid 20's (i.e., 20 times re-used). No sanitation related problems encountered. Then again, some yeast just don't track that well beyond 4-5 repitches.
One thing I would do, though, is taste that beer coming off the primary to be sure that it is totally free from any off flavors before pitching the second batch onto the yeast cake. And have a backup yeast ready (such as 34/70 sachets) just in case you need to use it.