Hmm...that is slightly more difficult...
Try making a list of the objects, then a Boolean (T/F) variable. In the events, have "when the mouse is clicked on (anything)" and keep the anything. In the "do nothing," replace it with "for all in order" and cycle through your list of objects. Do "if object under mouse cursor = item from list", then another if to test whether it is the first object or the second one clicked (change the Boolean after the first click). Then, at the end, store the objects in two separate variables and compare them.
In order to make sure that the user cannot click the same object twice, you might need to use a numerical variable to keep track of which list number you're on, then remove that list element.
Hope that helped!!!