I don`t think that is legal according to the rules. It would also be a bit timeconsuming, since movement (and everything else) is done in turns. It would take two turns for the transported country, and one turn for the transporter.
You could of course make your own houserule.
I agree; it would either require two turns, or allow a fleet to potentially convoy two armies a turn (once on its own maneuver phase, and once during the "allied" country's maneuver phase). Neither version strikes me as immediately palatable.