I think there's a rule that such a character combination must be named Master Blaster.
Yeah. There was this one time when my characters where on a time limit, and they needed to get to the castle of Ages, but they couldn't get Beyond Thunderdome.
Im so sorry. Im so sorry.
Actually, there is another way to get to this problem:
Animal companion rules. If I have understood them correctly, it's possible to get an animal companion to a state where (1) it's large enough to be a mount (2) it's had an ability score increase to move its Int to at least 3. At which point it is (stupid) sapient life. Or you could cast Awaken on a horse or some such.
At which point we're back to: It has its own actions, it can act on its own, how does this work?
This is much easier, since both characters are being controlled by the same player they can synch their actions to work together without the "no! i want to take this action or no i want to act this turn!
IIRC, animal companions with intelligences raised to 3 or higher are still not sentient. There's a blog on it somewhere around here.
What about a Paladin's mount which has an int of like 6. For laughs let us also give it a headband of intelligence and a couple of level bonuses to int. It could potentially qualify for combat expertise, something that most fighters can't do :).