In the send box
Actually I think that's still in Tools/Pay to Many, not in the normal Send box. At least that's how I've used it for years, if I want to have on transaction that sends to multiple recipient addresses.
Also you can use ! (exclamation mark) as amount for the last recipient if you want to send all the rest of the coins.
I've never used payment address (request for payment), but maybe this helps: with View/Show Addresses you'll have a new tab that shows more addresses in your wallet; you can pick one with no transactions and may do the job...