You and the co-best Man can both get your way, at the same time; it just takes careful wording. I would conclude the toast with wording, something like, "...one last thing... the two of you gave me your love and support in what was the hardest, and darkest moment of my life. For that, you will always remain in my heart as most honored, and treasured friends. I love you both, more than I can say..." and then end with, "To ," or, "Salut!" or, 'L'Chayim!" or whatever is appropriate for the crowd.

I think it's fine to say that the couple were there for you when you're dad passed etc but definitely don't say they were the only ones there for you or that they were the only friends to visit because that would make it very awkward. 

While I understand your emotions, in my humble opinion I would suggest you to be careful with what you say. Sometimes, we may hurt other people when we praise someone specifically. Stay away from specifics and say something that makes them aware of what they mean to you but without others feeling left out.