Gary,
If you're looking at a one or two man shop you won't have any worry with pressure drop in your system as described. Adding a second air tank is going to give you more storage volume (so does your header system), which only delays the start of the compressor as you use air, but then simply takes longer for the compressor to refill which gets your compressor hot. If you're painting your enemy system heat, and a good system is going to have your compressor running less than 50% of the time to keep it from getting too hot and putting more water into your air. You should be fine with 15scfm @ 90psig, especially with a newer gun. I would not think you need a second tank. Check your tools for flow capacity and compare to your compressor flow. On your piping, run your first line up from your tank, and slope the horizontal header back towards your tank, taking drops down from that header to your work stations. An automatic water drain on the tank is nice, but if you open your drain several times a day during heavy use it will work, too.