Does anyone know how this set up will work?

Lets say I have 2 servers that are running Ensim to manage domains. And I am reselling web hosting to whom ever signup as a reseller with me.

Server 1 has 400 domains on it out of which 10 belong to Reseller A. And now that second server has been added so I span 401th domain on to the new Server.
Now Reseller A wants to add another 10 domains. So he will have 10 on server 1 and 10 on server 2.
How would he manage all of his domains under one control panel? Does Ensim or Plesk have that functionality?

Both servers are with the same Hosting Company.

I'm pretty sure Plesk does NOT have that functionality. Not sure about Ensim.

Does anyone know if this type of account spanning will work across multiple cpanel servers?

CPanel would be better with this.

Is this something built into clustering? Would an account of the same name need to be on both servers and how would that work with DNS and everything else to place a resellers account on both servers?