How to show multiple camera angles together in a Multicam clip in FCPX?
What you need to do is mark the edit points for the clip where you want the effect to appear. Then highlight the edited clip so that it is highlighted in yellow.
Now copy that clip by pressing Command+C.

Now menu over the clip and select “Lift from storyline”.

You will now have an empty grey clip on the Storyline.

Click on that Grey clip and paste the copy of the original clip on to it.
Go back to the start of the clip and then change the angle of the clip on the storyline.

Finally, change the size of the two clips to give the side by side effect.
Dependent on you snap options you may need to drag the clips back together to the right places, when you paste the duplicated clip onto the stroryline.