Q: Can I have more than one file share witness in a cluster?
A: No, and you really wouldn't want to. The file share witness provides an additional vote (which is allocated dynamically in Windows Server 2012 R2 only when there are an even number of nodes running in the cluster) that's used in the event that the cluster splits and the file share witness vote is needed to enable one partition of the cluster to achieve majority and therefore quorum. If you had multiple file share witnesses, it would defeat the point of this arbitration and actually complicate the process.
If your concern is if the file share witness is unavailable, such as because of maintenance, the first thing clustering will do in an arbitration situation is attempt to online the file share witness (e.g., if it had been down because of maintenance, which would resolve any issues related to it being offline because of an earlier maintenance operation).