You will need to create a workplane on the face, then use construction geometry to project the outlines of the different edges of both parts onto this workplane.(It is important that you set your catch to allow you to select parts) Then, you will need to create a 2D profile that connects the important intersections of the projected construction geometry. This will define the mitre. Because of the step in the frames, this mitre will have two angles. Add additional geometry to the mitre in order to create a closed profile, then punch away the excess material from one part, then modify the 2D geometry on the same workplane to create a new closed profile (keep the miter part) to allow you to punch away the material you do not need from the other part. Because of the steps in the top of the part, you will have a 2 angle cut in the bottom. You need to decide how to handle this.
