Disclose everything to your partner and check with them.

They said that you can date whoever you want, then they should be fine with it and appreciate the transparency.

If they’re not okay with it, you can discuss it to understand each other and reach a common ground (which, if they are actually not okay with it, probably means not dating the other one, or breaking up, the other options are generally not a good idea).

Overall, just communicate, as that’s the core of a decent relationship, and do not let disagreements break this communication, and you’ll be fine.