I think Aries’ suggestion is beautiful, a great way to bring both families together (assuming they get along to begin with). However, Aries, that does not mean that marrying an only child,
especially if it’s the son, is not potentially without foreseeable problems, and not something to be considered carefully. For example, how dependent are the parents on that only child. How tied to the son is the mother. What happens after 120 years and there is only one child who can take care of a surviving parent (if and when necessary)? Nothing is undoable, but it should be considered seriously among the other things one considers when making a shidduch.