Off the top of my head, when you put two cogs on an AW you want them 3 teeth apart, so something like 20 and 23 is good. That splits the hub gears nicely. If you did the same rat-io in front, so 40 and 46 for example, you would get the same effect. Do that jump again, so 40 46 52 and you would double two gears and add one more. Mostly you would just add confusion. The best chain tensioner for this purpose is a rear derailleur. But really, the best thing to do is to use two cogs on the hub. This is easy and indeed much simpler than what you propose. Either way you need a rear derailleur, so why bother with a front one?
Indeed. Make sure you use the old-style dished cogs as you need only place them back to back to get clearance between the cogs. If you use the modern undished cogs, you'll need a spacer between them.