How these two meet
You are both unreachable and you do it in opposite directions, and the first year of this was the most interesting year either of you had ever had.
Scorpio is unreachable downward. There is a whole floor below the one you are standing on, and access is granted in pieces, on a schedule Scorpio does not publish. Aquarius is unreachable outward. There is no floor below, there is a person standing one step back from the entire proceeding including their own part in it, and the step back is not a defence, it is where Aquarius actually lives.
Two fixed signs, so neither position is going anywhere. Mars runs 2.13 years between reversals and gives no warning of the turn. Saturn spends more than a third of its time apparently going backwards and arrives in the same place regardless.
Both of these signs were handed a second ruler long after the wheel was finished, and this is one of only two pairings where that is true of both: Uranus, found in 1781, and Pluto, found in 1930. Neither body was visible to anyone who built the system you are standing in, and neither can be seen with the naked eye now. The two of you are being described, by everybody including yourselves, using instruments nobody had.
For Scorpio, the work is giving something away on no schedule, unearned, at a moment you did not pick.
For Aquarius, the work is stepping in rather than back, once, while a thing is still happening instead of after you have understood it.
And a third: neither of you is going to become reachable. What you can do is say which direction you are unreachable in, so the other one stops searching in the wrong one.
The question
How would either of you know if the other one had left?
One conscious action
one direction, named
Each of you describe, in one sentence, where you go when you go. Down, or out, or somewhere neither of those covers.
The other one does not follow. They simply now know which way to look.
The classical rulers underneath are Mars and Saturn, which is to say a body that holds for two years and then turns without warning, and a body that never turns at all and arrives anyway.
The distance is not going to close and neither of you wants it to. What has never been agreed is which of you is allowed to say out loud that it is there.