OP, the very point of not being "exclusive" is that you are free to date or have sex with other partners. If you were not free to do so, it would be exclusive. And then having the words "non-exclusive" and "exclusive" would no longer make sense.
So if the status of your new "relationship" is "non-exclusive" then this is not an issue at all you should be concerned about. However, you seem to have already decided to be "open" about what you are doing with both men. That is your decision, but in reality you do not actually have to be. You only met this new person within the last two days, correct? That, by itself, means he really does not need to know about any of these details. This is your life, not his. Whether he will be a large part of that life is still unknown. You said the man you have known for a while is already okay with it. If you had been "seeing" this new person for a substantial period of time (and I'm sorry, but two days is not substantial), then perhaps this would be different. At this point, you do not even know if this will last. You are perfectly in the okay to keep this to yourself if that is what you wanted to do. Honesty being the best policy is not always the case, but it does make a nice rhyme. Sometimes it is better to keep things out of the knowledge of others when it serves no purpose to share.
Sex does not have to mean relationship, and it can be casual. Based on your post, it seems evident you are approaching both of these encounters in a very casual way. The person you have known for a while is okay with it. The new person is not exclusive. There is no issue here at all.
Whether you build a relationship with this newer person or not is still an unknown. However, again, this is a choice you have to make on your own.
I only offer this advice because you asked.
Good luck in what you decide to do.