Each stake account may only be used to delegate to one validator at a time. All of the tokens in the account are either delegated or un-delegated, or in the process of becoming delegated or un-delegated. To delegate a fraction of your tokens to a validator, or to delegate to multiple validators, you must create multiple stake accounts.
This can be accomplished by creating multiple stake accounts from a wallet address containing some tokens, or by creating a single large stake account and using the stake authority to split the account into multiple accounts with token balances of your choosing.
The same stake and withdrawal authorities can be assigned to multiple stake accounts.
Two stake accounts that are not delegated and that have the same authorities and lockup can be merged into a single resulting stake account.