Any component added to a circuit adds to the resistance of that circuit. Adding a Ford solenoid in series with a Chevrolet solenoid, except for a safety feature, is a band aid at best. If the engineers at GM thought that by adding a Ford solenoid in series with a GM starter and solenoid would work better I'm sure they would have done so years ago. There are a number of possible causes if the engine won't start, starter won't spin, and they are:
1 Bad or loose grounds
2 Corroded cable(s)
3 Too small gauge wire running from the battery to the solenoid
4 Badly worn brushes in the starter
5 Arced contact in the solenoid
I have encountered all 5 of those things in my almost 60 years of messing with cars and trucks.