Co op with two separate accounts?

My partner and I have bought PoE2 early access on Steam and have been enjoying playing on PC together. I then purchased an access key on PS5 so we could play together when travelling for the holidays.

I was able to connect my Steam and POE account, and then link that to my PS5, but now my partner can't log in to his POE account as player 2. He is logged in to his Sony account on the second controller.

What are we missing? I thought we could play the same characters across multiple platforms - otherwise I wouldn't have purchased a separate key. We have no interest in creating new characters under my account.

Help appreciated, or feedback that this needs to be made clearer as a limitation if this can't be done.