I found the first sentence misleading---it sounded like one Stanchion ran on every node, and each node was configured to talk to its own Stanchion. It's not until the second sentence I understood what you meant. How about swapping them, and rephrase the old first a bit.

"Problems can occur if all Riak CS nodes are not configured to communicate with a single Stanchion node. Although Stanchion instances may be installed and running on each node, only one may be actively used by the cluster."