In your post, you point out how the 2 weaker players usually get beaten by the stronger one. Some good teamwork tips:
Build Trade routes asap. Caravans between yourself and an allied team are highly profitable, and endlessly renewable.
Use the signal flare to coordinate attacks. By brining your combined weight into a fight at once, you can overwhelm is defences, wheras if you let him take on your forces piecemeal, he can recover/reinforce between battles.
As a sidenote, have you considered not teaming up with Mister Backbiter, but teaming against him? If you can cooperate with the stronger player to demolish your reluctant ally, and make him withstand the brunt of the assault, you may be in a position to win the whole game shortly thereafter.
On the flip-side, always hold out some reserve forces when cooperating in an allied battle. Uncommitted forces will make your ally think twice about hosing you after a victorious conflict, and give you the option of doing unto him.