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Since posts are made via UIs, the options available for choosing post payouts depends on UIs supporting the available options - for example steemit.com does not currently support splitting payouts between multiple beneficiaries, yet it can be done at the blockchain level and is done by other UIs. I thought that, as with beneficiaries, more options existed for SBD/Steem splits than is visible in the Seemit UI - but maybe not.

but maybe not.

yep, not.

Also

Since posts are made via UIs

not necessarily true. You can interact with the blockchain directly via the cli_wallet, if you wish...