I recently added a second toilet to my house. I added the necessary vent line and kept the drain line to the correct slope. The problem I have is that the level in the bowl in the new toilet will drop after an hour or so. The new toilet is taller than the old toilet (by about 3-4"). I have found the level in the new toilet to be equal to the level in the lower toilet (as measured from the floor up). Could this partial drain down be due to the higher toilet equalizing with the lower toilet?

What is the space relationship between the toilets and how did you add the vent? Toilets are not connected via water so they will not equalize. However, improper venting can allow some of the water to be sucked out. I would flush both toilets and then mark the level with a pencil and don't flush. If the water level drops again, you may have an internal crack that is, fortunately, running down the drain and not through the floor.