First of all I am going to apologise to the op for monopolizing her post and also say that I agree with others that when learning to post just do the posting part and not worry about diagonals, that will come with learning and practise.
Then I guess I am going to play Devil's Advocate and go from there. I will take TXhorsemans suggestions about using up down instead of beats. (his suggestions/ideas are always good)
(up/down)=one stride=one post=2 beats, right?
I am posting and want to change diagonals
(up/down), (up/down), down, (up/down), (up,down)
the only place I sit an extra beat is the singular down
when I am posting in time with the left shoulder and want to change diagonals, when I come back to the saddle, I hesitate (don't go up with the left shoulder going forward) and then rise with the right shoulder, so I don't really feel two beats. My back is really bothering me today so won't be riding but as soon as I can, you can bet I will be out there posting around checking for double beats