[avaserfi]

There wouldn't be a problem with it, but there really isn't a point in doing that unless you need more output. I would doubt you would need more output with an ultra unless you have a massive room.

The real reason to have multiple subwoofers is for placement in regard to getting a more even bass response. Placing a sub on top of another will not achieve this end but instead just allow for more output.

So the real question is why do you want another Ultra and what are you expecting of it?