ethereum intentionally split by banning opposing nodes (google it: --oppose-dao-fork)
It didn't ban nodes. That instruction simply installed an ETC client or an ETH client. With a different protocol. It is as if you could use a bitcoin core client, and give the instruction --run-litecoin.
And transactions from one chain WERE transmitted to the other chain, which caused a lot of surprise. But this is normal: miners wanted the fees, so they went LOOKING AFTER valid transactions (not on the network, but on the block chain !).
lol bitcoin and litecoin are different coins. etc and eth are different coins. they have their own networks.
they have their own mempools.
they do not intercommunicate. because they only connect to the side they like and ban from talking to the side they dont like. to avoid the orphan drama.
There is no orphan drama when there is a hard fork. Blocks on one chain are simply invalid on the other one, and respective miners build on one or the other.
There is no NEED to communicate, but it is not the fact of stopping the communication that makes that there are two chains of course. The double-valid transactions WILL get across (miners will pick them from the other chain) and the block information is not relevant because mutually invalid.
butcoin and bitcoin will also be entirely different coins, like ETC and ETH. And yes, most probably their clients will end up only talking to one another within their network, but it is not this "banning" that splits the chain.