In my experiences (English/Spanish), we end up speaking mostly Spanish with random sentences in English thrown into the mix. Sometimes, the other person will speak fully in Spanish and I speak fully in English. It really depends, but there is usually a lot of overlap.
I have two friends who do this in Norwegian and German. They each speak their native tongue to the other
Either one would be fine. Native Polish and C2 English, so it works with both.
Switch between languages with every sentence to confuse everyone around.
Both, switch them up when necessary for clarity.
In my experiences (English/Spanish), we end up speaking mostly Spanish with random sentences in English thrown into the mix. Sometimes, the other person will speak fully in Spanish and I speak fully in English. It really depends, but there is usually a lot of overlap.