Why don't you ask everybody some questions, first - yourself
What do I want to be able to do to express affection to each of them?
Maybe I'd ask "primary" next since it seems to be them you are worried about. Hey I wanted to ask you about PDA, this is what I'd like to be able to do, are you uncomfortable about any of it? (or comfortable with MORE than I mentioned?)
Then ask your other partner Hey I wanted to ask you about PDA, this is what I'd like to be able to do (tailored, I imagine to include or not include things your other partner already felt OK or not with, and anything they may have added to your initial idea about "AOK with me activities")) are you uncomfortable about any of it?
Then before you get together, let everybody know what to expect from YOU as a shared partner, so there's not going to be weird "should I shouldn't I" issues and you can just enjoy the time together in as relaxed a manner as possible.
Happiness will never come to those who fail to appreciate what they already have.