If you have two objects and one of them has an oscillate behavior on the opacity to blink it on and off (Square Wave Shape) and:
you add another object and add a Link parameter behavior to the first object's opacity value, to make it blink off when the other object blinks on (and vice versa) :
go into the Link behavior, and for the Scale parameter, either click and drag on the Scale value to set the value, or double click on the Scale value and type in: -100.
I also needed to set the Apply Mode to Add to Source, then this worked. It's one of those things where Apple has abstracted the function so much that it isn't discernable. I slided the slider to minus one but that was as far as it would drag. I assumed that is were I would have made the invertion but not being able to get there I didn't follow up on it. I knew that you can go outside the silder range but usally they go equally positive and negative.
I would like to see the whole expression (and be able to edit it).