As far as I know there is nothing preventing Mecanum wheels from working in pairs.
For example you could have a 8WD with the front two sets acting as a traditional Mecanum set and the back pair acting as a Mecanum set. In this set up you would have two X's (from the top side --
see this thread)
\.../
/...\
\.../
/...\
The first and 3rd wheels should be driven together and the 2nd and 4th wheels should be driven together.
A problem will be do you lower any of them? I suppose that if you do, you'd lower the 1st and 4th rows otherwise you'd effectively have the middle pair on the ground forming an O from the top. Which is bad because you won't be able to spin effectivley (again
see this thread)
Another alternative is to have this config:
\\.//
.....
.....
//.\\
BUT... The more I think about it, the more I believe the question is the wrong question.
Wrong question: "Has anyone tried stacking Mecanum wheels?"
Right question: "How can I go sideways with Mecanum wheels more effectively (= fast and straight)?"
The answer to that I think is two fold.
#1 keep your CG in the middle of the machine (as much as you can) and
#2 feedback loop using an angular rate sensor*
With these two steps I believe you can get what you want without having to resort to dual Mecanums.
Cheers,
Joe J.
*often called a "gyro" for historic reasons involving the means that angular rate sensors were implemented prior to MEMs electronics. Like the confusion between "power" and torque, I wish the world were otherwise, but I will just add it to the list for the time being and move on...