Is sitting between two people in the first row on Friday before the rows expand considered a separation between them, and does this result in the loss of the reward for Friday prayer, and what is the ruling on repeating this action in the future with the intention of joining the first row when the prayer is about to begin?

There is no harm in separating two people if they give permission. Separation includes stepping over them or sitting between them. Stepping over is disliked by most scholars, especially on Friday. However, if there is a gap that can only be reached by stepping over, there is a difference of opinion. As for sitting between them, it is permissible and even recommended if it does not cause harm, but rather to fill a gap; otherwise, it is forbidden unless they give permission. The Prophet, peace and blessings be upon him, said: "It is not lawful for a man to separate two people except with their permission."