A coinbase transaction is allowed to have as many outputs as the miner wishes so long as the total amount in those outputs does not exceed the block subsidy + transaction fees for that block.
Thus a miner could create a coinbase transaction which has more than one output which pays to himself and someone else. It doesn't matter who the recipients of the outputs are. In fact, it doesn't matter if the outputs have invalid scripts. There are no rules on what the output scripts must be.
The amounts of each output are decided by the miner creating the coinbase transaction. They can choose to set the amounts as they wish. The only rule is that the sum of all of the output amounts cannot be larger than the reward (subsidy + fees) for that block.
While each block ostensibly has one miner, miners will typically team up in mining pools. This is effectively multiple miners mining one block. In reality, it is a single miner in that pool who gets lucky and finds the valid block, but the pool as a whole shares the block reward.
Some pools will pay out all of their miners with the coinbase transaction by creating multiple outputs that pay each miner in the pool. The amounts are determined by whatever the pool operator and miners have agreed for the distribution of the block reward.
