How do you wear two army achievement medals?

You would wear one Army Achievement Ribbon with an oak leaf to indicate each subsequent award affixed on the ribbon, or the ribbon portion of the medal.

This is correct. To specify, one would wear a Bronze Oak Leaf Cluster for each subsequent ribbon awarded. The Silver Oak Leaf cluster would be used to show 5 subsequent awards. For example:

One is awarded the Army Achievement Medal 11 separate times. Therefore one would wear The Army Achievement Ribbon with two Silver Oak Leaf Clusters.