The correct and most commonly used phrase is "happy for both of you."
For example:
- "I'm so happy for both of you on your engagement!"
- "We are happy for both of you and wish you all the best."
The phrases "happy for the both of you" and "happy to both of you" are less common and may sound awkward to some native English speakers. It's better to stick with "happy for both of you" when expressing your good wishes and congratulations to a couple or group of people.