"When I add water to the bowl, it goes down to the new lower level rather quickly. Does that help in problem determination? I can tell the level in both toliets is too low based on the ring left by the old water line that existed previously."
One of the rules of failure analysis is that when you have a problem it is usually caused by one failure. Not always true, but that is where to start.
First, I must assume that your observation is correct, that the water in two toilets goes lower than the normal static level even when you slowly add water to the toilet bowl.
Now, what ONE failure would cause the level in two toilets to go low in a static condition resulting from gently pouring water into the bowls?
The only thing I can think of that would cause that condition is if there is low pressure (lower than the pressure inside the house) on the downstream side of BOTH toilet traps.
I assume that you don't have some great blower blowing air into your house in the winter time, and the difference in pressure due to the vent stack being warmer than the outside temperature is negligible (I calculated that to be sure.).
The only thing that could allow the pressure on the downstream side of the two toilets to be lower than atmospheric pressure is that the vent is plugged. If you are a Do-It-Yourself person, you might take a garden house with a straight nozzle up on the roof, adjust it to the most forceful jet spray, and run it down the largest vent pipe near the toilet. Push it down, with the water turned on, for maximum effect. If the vent is REALLY PLUGGED TIGHT, the pressure when the water comes to the top will almost certainly force it through. On the other hand, if the blockage is below where another vent is teed into the stack, the water could bypass and may not unplug it.
Now I have two reservations about this analysis. First, for this low pressure condition to persist after flushing, the vent must be really plugged tight. Second, something other than a blocked vent is necessary to create the lower pressure on the downstream side of the toilet. I haven't been able to figure out a reasonable scenario for that.
Is it possible that you have a pumped system of some kind, or that the municipality has some kind of pump station or vacuum collector sewer system in your neighborhood? That kind of sewer, in conjunction with a blocked vent, could cause the condition that you describe.