Make a deal with TutaNota for ProTut encryption keys between you two.
It's really simple.
People who are paying for Tuta are not (or rarely) also gonna pay for Proton.
And vice verse.
The most logic thing would be to not make things complicating or punishing, and just call Tuta, and make up a set of keys for trafic between the 2 of you.
Right now consumers still have to tinker with passwording email for the travel inbetween, while both inboxes are encrypted. And this is funky at best :)