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. Duplicate tx from MPcoin get discarded.
Forking without consensus will most certainly lead to both chains/coins become completely worthless.
Forking without consensus is exactly what will happen if volumes suddenly ramp up filling the 1MB blocks and everyone is yelling about ridiculous delays, bad press, collapsing price, etc, and a large number of nodes feel it necessary to increase the limit, while others refuse to, welcoming the "necessary pain".
That is why Gavin has proposed re-using the block version majority process, which worked very well in 2013 for P2SH, and at least gets a big consensus in the mining power. A well-executed fork, done over many months, will be very smooth for all users.
LeMiner, fully agree.