First, feather edge wear is a normal wear condition. It is caused by the fact that as an indiviudal tread element enters the footprint, it is pushed back and the leading edge gets rounded.
When the tread element leaves the footprint, it holds on until the friction is overcome and it snaps our of the footprint, leaving a little "feather" on the trailing edge. This is commonly called "Heel and Toe" wear because the result looks a bit like a human foot.
This type of wear can be aggravated by mis-alignment - toe in particular. Keep in mind that the ideal alignment condition is for the tire to be pointed straight ahead in the dynamic (rolling) mode, any deviation from straight ahead causes a slight angular force across the face of the tread, accellerating the wear rate. That means that even toe-in values "Within Tolerance" can cause the condition to be more severe.
But toe-in also aggravtes other tire wear conditions. "Scalloped Wear", "Cupping Wear", and "Diagonal Wear" can literally all be thrown into the same bucket as they are either different ways of expressing the same type of wear. Personally, I like to use the term "Irregular Wear".
Yes, shocks can contribute to these conditions, but my experience says that alignment plays a larger role.
The fact that your cars are wearing different edges would point to difference in alignment as contributing to the conditions.
How to fix? Without more details, it's hard to be precise. Part of the problem is with the alignment specs themselves. It is pretty common for alignment specs as published by vehicle manufacturers to have pretty wide tolerances - wider than they should be to assure good tire wear.
Further, some vehicle manufacturers have alignment settings that produce good handling, but aggravate wear conditions. Camber is one such setting. I've found that anything over a degree tends to generate such wear.
But part of the problem may be such simple things as rotation and inflation pressure. Once an unusual wear pattern starts, it is very difficult to "erase" that pattern. The best practice is to rotate regularly to prevent those types of wear patterns from developing too far that they can not be undone by the wear pattern generated in another wheel position.
So I would get an alignment and insist that all alignment values be within the inner half of the spec.
Second, before you get the alignment, find out what the values are supposed to be - including the tolerances. In order to help quantify when the target values are too high, I've come up with some "Idealized" settings:
Unfortunately, once such wear starts, it's too late to see if the corrected alignment fixes the problem. What happens is that the wear just proceeds more slowly. You can't gauge whether or not the corrected alignment prevents the problem without also starting with new tires.
Aggravating the alignbment problem is that most alignment techs don't have enough theory to know when the alignment specs are at fault - so they blindly follow the published specs. So it becomes important to do your homework on the alignment settings first, then find an alignment tech who will follow YOUR desires, and not what is published.
