To avoid a single point of failure on the backend SAN system, can I place one quorum witness disk on each site and have the cluster use both?
No you can't. The whole point of witnesses is to avoid a split brain situation.
The solution ended up being a file server witness placed at a third secure location with direct links to each datacenter to maximize redundancy.
If we just have 2 x nodes in a failover cluster then if they can't communicate both will assume the role of the cluster - the Split Brain Morten mentions. the quorum disk fixes this as the node that can see the quorum disk wins as it gets 2 votes compared with the other node that just gets one. If your quorum disk fails you'll still have your two node cluster so don't worry about that unless a node fails at the same time. You do mention this disk is on a SAN so that does seem unlikely. More on this here http://blogs.msdn.com/b/clustering/archive/2011/05/27/10169261.aspx