block_1 - antpool mines this altcoin block, because they mined bitcoin block_1
block_2 - pool-x mines on top of block_0, because they will check last 144 blocks and see antpool already mined say, 20% blocks. top 50% pools consist of 3 pools; antpool (20%), pool-y (16%), pool-z (15%).
antpool really gains by splitting into 2? I am not arguing. just clueless.
This is an altcoin, with its own consensus and enforcing nodes.
They don't even need to split anything physically, just put some random text into the coinbase TX and each block will look like it's mined by a new pool. As frodo explained above - the reason we know who mined a block is because most pools identify themselves voluntarily. If there was a penalty for being big we would see 144 different pools mining those last 144 blocks and would have no way to determine the bottom 50%.