Maybe you're looking at it too generally, Tom. I got into this in the "impossible..." thread.
Knowing something is unstoppable shows a definition or knowledge of its interaction with everything else in the universe.
They would have to be of the same material, or have no awareness of each others' differences, so that in effect they WERE unstoppable and impermeable.
OR
The wall would have to constantly change to accomodate new materials and prevent breeches, where the "cosmic arrow" would do the same, so they cannot be of one material, unless the material allows pass-through.
By this definition they would pass through slowly as each found a way to bypass the other, granted any change in composition took at least the smallest amount of time.
I would just as soon say, since it was never defined, they could meet, but only at location, and the time would be different for each, so one couldn't exist in the way of the other.
interesting and thought provoking