Yes, that was my logic but since than I tried with just single one and it is solid like a rock. Also keep in mind the pictures on their site are miss leading a bit and you can grip the router much higher for very long travel if needed.
And there is nothing wrong with conversion plate, if you search forums there are plenty of users that did this approach for various router mounts and Z axis actuators.
ninja edit:
You can see in my monster carve thread how I used the mounts: