So, in this question, a process for casting a spell while you're in the middle of casting another spell is described, and the consensus in the answers is that it works, and allows you to cast a second spell while you're in the process of casting a second spell.
What I want to know is how this affects the passing of priority. Normally, when you cast a spell, you complete the process of casting a spell, and it is put on the stack before passing priority to your opponent.
However, in this case, you interrupt this process by casting a second spell, which would normally require you to pass priority to your opponent before the first spell's casting process can resume, right? How would the fact that you have a partially-cast spell affect the priority passing?
Would your opponent be able to affect the first spell with counterspells before you've finished casting it? Does your second spell become uncountable because you can't pass priority until the first spell has finished being cast? Do you finish casting both spells before passing priority with both of them on the stack?