You might consider cutting them to length first and then lining them up edge to edge and running them thru the table saw cutting both at the same time
Also setting stops on your miter fence will do the same thing. (Maybe you tried that) Best way is to set the stop ,cut both pieces, then reset the stop for the next and cut both,and so on.
You could also use a router by setting up a frame jig on the workbench to hold the pieces then clamp a straight edge to guide your router across where the slots are to be cut, you would have to have the right diam. bit too.
Hope this helps.
Herb