You're trying to find a center between three locations? Like find a meeting point between three people so that they all have to travel the same amount of time?
Sort of, but I'm not looking for one point, I'm looking for an area.
Actually, I've kind of found a solution using that second link in my initial post. I'm still skeptical of the results that site produces, and how it calculates drive time, and there are other problems with its output, but I have a rough result that's somewhat acceptable.
Basically, I ran one address through that "How Far Can I Travel" site and exported the resulting KML file. I then did that for my second point. Then, I was able to load both files into Google Earth and find an overlap. I've attached a screenshot of the result.
The problem is that because one of the two addresses is within about 10 miles from the Potomac River, there's no way anyone could drive to the other side of that river in 30 minutes given how the bridges are spaced*, so the area is skewed and doesn't go up far enough.
But mostly I'm not satisfied with the results that site gives...
*it's surprisingly far between the American Legion Bridge, which connects MD and VA on the beltway, and the next bridge west at Point of Rocks. About 29 miles as the crow flies. In between there's a ferry, still pretty far out.