Diagnosis
You're probably not in the correct viewing mode. The V43 doesn't auto-split; you have to manually select the split-screen view after both cameras are paired and online.
How to Fix It
Make sure both cameras are powered on and showing a solid green light. On the parent unit, from the main single-camera view, press the 'Menu' button. Use the arrow buttons to navigate to 'Camera' or 'View' (the exact wording varies). You should see an option for 'Split Screen' or a double-square icon. Select it. You should now see Camera 1 and Camera 2 side-by-side. You can usually toggle which camera's audio is active with another button press.
If That Didn't Work
The second camera might not be properly paired to the monitor as 'Camera 2.' You need to pair them in sequence. With the monitor on, press and hold the pairing button on the BACK of the second camera until its light flashes. Then on the monitor, go to Menu > Settings > Add Camera and follow the prompts. The monitor can only handle two cameras total.
Heads Up
In split-screen mode, the video quality for each feed is reduced because the signal is being divided. Don't be surprised if the image looks a bit more pixelated than in single-cam view.