For iMovie 10 and above:
Drag and drop the clips that you want to show at the same time to the timeline and make sure one is above another in the timeline.
Click the Video Overlay Settings icon above the Viewer window and then select Split Screen option from the drop-down box.
Select a clip or range that you want to show at the same time as
another clip, and drag it above a clip in the timeline.
When you see the green Add icon (+), release the mouse button.
If the video overlay controls aren’t shown, click the Video Overlay
Click the pop-up menu on the left and choose Split Screen.
The split-screen controls appear above the viewer, and cropped
versions of both clips appear in the viewer.
To change when the split-screen effect appears, drag the split-screen
clip in the timeline so that it connects to a different spot in the
clip below, or drag it to a different clip. You can also drag the ends
to lengthen or shorten it.
To apply the change, click the Apply button in the split-screen
As for selecting the audio from the two clips, you need to detach the audio from the video and adjust their locations on timeline.
- In the timeline, select a video of interest
- Choose Modify > Detach Audio (or press Option+command+B).
Now it behaves just like any other block that you can move, overlap, trim etc.