I’m not sure I understand what you are wanting to do. It sounds like you want to mix up the scents in the box lineup for your NW class???
I don’t think mixing the birch scent and truffle scent in the same training session (or even context) is ideal. Lola won’t be asked to find a truffle scent in a NW setting and also won’t be asked to find birch in a truffle hunting scenario. I also imagine your alerts are a bit different (even if subtle differences).
I think they should remain separate activities. When you start adding multiple scents to a truffle dog training lesson, it will be multiple truffle scents. For example, setting out a white and a black and working on Lola’s ability to recognize both as acceptable finds. Surprisingly this can get tricky and is difficult for some dogs who “like” one species of truffle best. You will do that LATER down the line. For now, I’d keep the birch and truffle scent training within the context that they will be used.