I would think they do. I've never seen a general ruling in the Core Rulebook that states that a PC can't add a modifier twice to the same effect if each addition is coming from a different ability. Of course, you're only allowed to get both bonuses if you yourself are making the check, though you likely were doing that anyway.
It's good insurance too, because if you can beat the effective DC 25 (assuming you're using an advanced medkit), you heal HP equal to their level + your int modifier + twice your cha modifier + a boatload of dice more if you don't add your expertise die. Even if you somehow botch the check just enough to get between 20 and 25, you'll still heal their level + your cha modifier.