I think the best way to learn a two-beat in backstroke would be to start with a very small six-beat and gradually eliminate beats 2 and 3 and 5 and 6. The timing of the arms should take care of itself, I think.
Another way would be to use a dolphin kick, with the up beat on each hand entry.
Trying to swim with no kick at all might also be helpful.
Any other suggestions?