Have to admit I didn't even know SIP was a thing until now. 😄
Here's the workaround / correct setting to get it working like you want it to:
Settings -> Network & internet -> SIM card -> General settings -> Return call from call log
Make sure you have it set to 'Via preferred SIM card'.
That should enable the 'Call with..' prompt when you make a call from the call log, letting you choose your SIP account.
Making calls from the Contacts app or by looking for a contact / dialing a number in the 'Recents' tab should by default ask you which one to choose (SIM or SIP). Just tested it with 3 devices here. If not, please double-check your settings and clear the cache of the caller app.