[Edratman]

I have seen trading places, but only when I was using a small force, say 5 units or less, in an attempt to block the enemy army from moving. This was done in conjunction with an attack on the original enemy province with a larger force from a third province. Easy to remember because it always pisses me off. I've not seen it the other way, but that is probably because the AI won't attack with a force of 5 units.

I don't believe I've ever seen two armies passing without combat if both had at least 25 units (as a guess).