There’s one son of one mother and there’s the other son of the other mother. Is there are two sons valid?
It is valid. Odd, but valid.
Odd because being a "son" is something that is true of literally every male person. So we would not normally say "He is a son" because it is always true. Similarly "There are two sons" is always true when there are two male humans. We nearly always speak of "My son" or "her son" and not "a son".