How can I make a 90-degree corner with 2 frames? 

To make an inside corner, you need two or four Mini feet and two or three Inside Connectors 90 degree. 

See our Quick Guide : Connections for a simple overview of the connector.

To make an outside corner, you need two Mini Feet and three Outside Connectors 90 Degree. 

Instead of the Standard Feet you use the Mini Feet to make the corner stand freely.