Hi Peter - it's absolutely fine to have a 'Master of Masters.'
All you need to do to implement this is to set the server directive in your second tier Masters to point to your Master of Masters.
Your agents, per datacentre, will point to their datacentre Master, and have no knowledge of the Master of Masters.
Typically, the MoM will host the CA function for the second tier Masters, but this isn't an explicit constraint of the model
Cheers.
David Bryant Moore.