If the legends play for opposing teams in SPL it's going to be legendary match ppl will talk about it for years. Messi's enjoying his life in Inter Miami but they can't give mega contracts like SP & Al Ahli. He's 38 so it's his last chance if he wants SPL money he won't get another offer.
It's possible he'll reject moving to SPL because Cristiano's established there but Messi doesn't have to go to Al Ahli if his family doesn't want to move. His children's going to change schools & his wife's going to have no friends so if they're happy he won't move.
Yes, if both legends should play in the same League now that the comparison is very much, both players may lose focus, this is what teams are intending to do because it will raise more money for them, in a point when both teams will have to face each other and at the end of season who wins the league. But aside the comparison, it takes a lot to relocate as a player with family, lets not forget he has his child in the academy there in US. A player like Messi who has nothing to worry about aside enjoying the sports, should prioritize his family well being, his endorsement are already flying high cash for him, despite the huge offer from Saudi, he has enough to live with in his lifetime.