From my experiences a Union with a Twin Flame is not something that you aspire to or achieve. You already know who you are to each other whether you are separated or not.
In fact, it is the natural bond that you share — the Sacred Union — the energetic balance and inner harmony that you feel when together that makes it almost impossible to forget each other.
If you separate from a Twin Flame and then have doubts that he or she is your Twin Flame, there is a higher chance that you are not Twin Flames after all.
You are already in a Union with your Twin Flame.