You can say "I love you with all my heart". This isn't something that depends on how many people you love. It means you love them completely, fully, and you can love more than one person that way.
You can also say "you are so important to me" or something like that, that doesn't imply "first".
But honestly, have you never told someone "you're the best!" and then told someone else that as well? These are expressions, they don't mean you love your other partner any less. If you want to be specific, find something they don't share. Like, if one is blonde and the other a brunette, you can say "you're the most important blonde in my life" or something.