http://mms.mongodb.com/help-hosted/current/faq/monitoring/#can-i-use-two-monitoring-agents-to-connect-mongodbs-in-different-data-centers and http://mms.mongodb.com/help-hosted/current/faq/monitoring/#do-i-need-a-monitoring-agent-for-every-mongodb are a little ambiguous.

These should be clarified to say that only a single monitoring agent is required for each group. If you have multiple groups, then you need multiple monitoring agents.

This same logic applies to MMS Backup Agents. Only one agent is required per group, but if there are multiple groups, they need one agent per group.