IF the mirror mounts are metal and grounded, I'd put a single 4 ft on one mirror. The second one gains you little or nothing. This choice is for ease of installation.
If you want a little better performance, which really might not be desirable, put a single one on the ladder. If you do that, be aware of the new total height you will have and make sure your antenna flexes easily.
The point about not having a metallic ground at the mounting point is a good one. If you don't have a good grounded mounting point, the antennas designed to work without that are sometimes designated as "marine", since boat bodies are almost never metal.