Formal:
* Je les aime tous les deux. (This is the most literal translation and is appropriate for formal situations.)
* J'aime l'un et l'autre. (This is a slightly more poetic way to say it.)
Informal:
* Je les aime tous les deux. (This is still acceptable in informal situations.)
* Je les aime bien tous les deux. (This is slightly less intense than "j'aime" and is more appropriate for friends or family.)
Specific to gender:
* J'aime les deux filles. (I love both girls.)
* J'aime les deux garçons. (I love both boys.)
Specific to relationship:
* J'aime mes deux enfants. (I love both of my children.)
* J'aime mes deux amis. (I love both of my friends.)
Ultimately, the best way to say "I love them both" in French will depend on the specific situation and the people involved.