Using wet domes for split shots is difficult.
There are small holes either in the port or the lens thread, which allow water flow between the port and lens, whether you want it or not.
This means that when shooting over / under, you will likely have some water there, but not full or empty, which will result in another water line in your photo, in addition to the actual water line.
If the water is completely still, you can align both water lines and get a clear shot. But that’s very rare and only possible in a pool or extremely still water.
One method is to block the holes with tape, just for over/under shots, to avoid the 2nd water line.
You can also simply shoot quick, leaving it submerged so it can fill up, then lift and shoot fast, before it empties. Then repeat.
Another problem is that focus doesn’t always work out with wet lenses, so likely you will have either the top side of bottom side in focus.
Using a small aperture will help improve the sharpness of both sides.
Below – split shot taken with Fantasea FRX100 V + UWL-09 wet lens, in Cuba. The water was as still as a pool in the mangroves, allowing me to take this shot.