Acceptance is really the whole point of this.
There are some people who meet both their TF and a soulmate in the same lifetime, and I think as long as you are honest with yourself and them, you could have a happy relationship with a soulmate knowing that your union is not going to happen in this lifetime.
What I don’t think you can do is use this person as a placeholder. That isn’t fair and isn’t good karma, so that won’t serve either you or them. Your twin might live in your head but you can have a great relationship with someone else as long as you’re not lying to them about anything.
I also think there are people who are in FWB situations or something while they wait for their union, and as long as you’re not lying to them (telling them you love them and you only think of them, etc), you have nothing to feel guilty for.