A husband and wife have arranged for a double plot. The husband dies and is buried in his spot. The wife remarries and will not be using her side of this double spot. Can a stranger (male or female) use this empty spot?
He or she can use it, but they first have to ask permission from the lady in question, who has presumably paid for the plot, and in principle “owns” it.
It is therefore hers to give or to sell, and you have to ask her before using the spot.
For male or female burial see here.