I'll be honest your requirements are somewhat beyond my skills and I'm sure their are other more knowledgeable members in this group who will take me to task, But I know when Giz (from this forum) and myself tried two handsets on my last layout, Loch Dore layout we could never get two handsets to work on the basic 2 amp setup (you have renumbered the second handset?).
The problem was resolved by getting the NCE Smart Booster and a 5amp power pack and now no problems. I've never used power districts as i find the gaugemaster DCC frog juicers for points resolve any shorts that happen round the points area.