Welcome to the forum, Ashok!
Each human life is a different gandhabba.
But in the case of twins, the zygote splits into two and effectively become two.
Even if the two split cells are identical, there are no two gandhabbas that would be identical (no two humans are the same). Gandhabbas are “selected for a given zygote” based on how well gandhabba’s gati match the gati of the mother and father. That “matching process” selects the “best matching gandhabba” available at the time. This is why there are instances where a child may grow up to have very different gati compared to either parent.
In the case of two gandhabbas taking possession of the split zygote, while they are likely to have similar gati, they can never have exactly the same gati.
As the twins grow, they will start exhibiting their own “inherent gati”, and in some cases they may be quite different. Of course, their “original gati” will change towards some common gati too, since they will be brought up in the same environment.
The conception process is discussed in the post,”What does Buddha Dhamma (Buddhism) say about Birth Control?“.