You don't get it. Nodes of the different networks don't have to communicate. Hashes are the same on both networks so anyone who watches the transactions on one chain can broadcast them on the other chain.
No one cares about this. Duplicates from MPcoin get discarded.
Gavincoins are douplicates of Mpcoin - you can't discard them.
In case your hopes for the difficulty causing trouble won't come a reality you're toast with the Gavinfork.
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no matter how it plays out:
people will be better off using altchains that they can use with their internetconnection and HD's (in case Mpcoin doesn't make it - but if difficulty doesn't kill it, it will most likely be the surviving chain in the long run)