var pathToOpenFile = activeDocument.path;
var nameOfOpenFile = activeDocument.name.replace('.psd',''); // that removes a psd file extension if it has one
var secondFile = new File(pathToOpenFile + '/' + nameOfOpenFile + '_ambientOcclusion.psd');
this is written for psd files, but can be adjusted easily for other filetypes.Save that as a .jsx file in your scripts folder and restart photoshop if its open. In your action, where you need to open the '_ambientOcclusion' file, insert this script. This page has info on where to put the script, and how to insert a script into an action. Hope this helps with your goal.
I've written something like that already, but anyway: Thanks for help : ).
Maybe it will help someone else in the future. Next time you ask a question and figure it out yourself,consider updatig your post so others in the same boat can find it here.