Not an expert on this command, but I enjoy a challenge! ;D
I am with mac, as I am not sure a) what you are trying to do and b) how you are trying to do it.
Are you trying to call the onMapSingleClick command from within the "ratelo_artypos.sqs" and get the results in "ratelo_arty.sqs"? If so, as the "_pos" from "ratelo_artypos.sqs" is local to that script, it is not recognized in "ratelo_arty.sqs" without making it a public variable and then declaring it as such (I think).
If it were me, I would call "ratelo_arty.sqs" from a trigger thereby being able to use the _pos generated by the command. You could set the condition of the trigger to only allow its use after a varaible is called from the "control" source (in a script or different trigger, use "setartypos = true" and the condition of the "ratelo_arty.sqs" trigger would be "setartypos"). You could then put all of the hints/chats and execute the script in the "on Activation" line of the trigger like so:
Hint "Please select a postion for your artillery strike.";onMapSingleClick {[_pos,_shift] exec "ratelo_arty.sqs"};onMapSingleClick{}};Hint "Point was selected."
As usual, my replies are suspect until proven true as I am at work and cannot test the solution. With a litlle more info, I am sure someone can get you going in the right direction.
Wadmann