The word "son" is understood. Also, "second" is in ordinal form. Alternate translation: "the oldest son ... the second son" (See: [[rc://en/ta/man/translate/figs-ellipsis]] and [[rc://en/ta/man/translate/translate-ordinal]])
This can be stated in active form. Alternate translation: "so David considered them to be one clan" (See: [[rc://en/ta/man/translate/figs-activepassive]])