A twin is you in another body - they come in like magic, shake up your world, illuminate the parts of yourself connected to source while drawing stark comparison on things you have to work on.
You’ll go through a dark night of the soul that feels like painful agony from separation. You’ll scream at God (or who/whatever), feel like crying on the floor for days - and without this dark night, it’s not a twin. That’s the true test in my experience, especially when your signs have that intense karmic connection as mine did.
The runner feels things are too intense and may seek that connection elsewhere, but eventually learns it can’t be replicated anywhere but with twin. Meanwhile, the chaser has to reach that point of surrender, saying, ‘If it happens great, if not I’m okay.’ The hardest part is being able to honestly do that - because most of the time we say this and then we try to ignore the fact we’re just hoping that tomorrow the separation ends because we said it.
When you finally come together, usually when both decide they can’t live without the other, that’s union - and then you get working on mission.