First, split the clip at the point where you want the slide to start.
Right-click on the clip and choose the "Pan / Crop" option.
A "Video Event Properties" pop-up window will open.
In the top right corner, you will see buttons "Begin" and "End"
In the main window, you should see the first frame of your clip with a dotted-line box around it.
Make sure the dotted line box is completely around your video frame, and the button "Begin" is pressed down.
Then, click the check-mark box next to the "End" button. This should also cause the "End" button to be pressed down.
Next, click-and-hold the dotted-line box in your pop up window and drag the box to the left. --This sets where the viewable frame will be moved to by the END of the clip.
(The box represents the area that actually gets displayed on screen, so if you want to have your movie appear to slide to the right, then drag the box to the left. This pushed the movie image to the right side of the box (or "Frame").
To confirm everything is set right, click the "Begin" button to see where the frame will be at the 'beginning' of the clip. Then, click the "End" button to confirm the frame box has moved to where you want it. This is where the frame will be at the 'end' of the clip.
When you play back the clip, the program will automatically (and gradually) pan the clip based on how you moved the frame.
You can then set your second clip to play in another layer -- and use the same Framing property to crop or pan the second clip on the left side.
Hope that helps -- let me know if you need better clarification.