Oh, god. This is perfect. I was so excited when you told me you were writing these two together, and it's even better than I could have imgined.
You have both their voices down perfectly, particularly Tom's, in sentences like "I find I believe you." and "Except that I did. Like last time. Poland. I knew, but didn't entirely believe."
And the whole Germany/Poland thing fits in so nicely with bookshop!Remus (which is one of my most favorite Remus situations) and I love the way they come together and the way they both accept, in their sad, lonely, quiet ways, that it's just to be a one-time encounter, and how they both take it for what it's worth because both of them have learned about loss.
The man was slim, pale, brown – rather like Remus himself, even down to the pained wariness in his eyes, although when he turned his head, his spectacles caught the light and hid them. I think that's my favorite sentence in the whole thing. It says so much about Irwin's character in so few words.
Such gorgeous controlled vulnerability, and you show it so well. I think I've described your writing as being like jigsaw pieces before, and that's true again here, everything fitting together to make a beautiful, surprising picture.