Right, so "Essence losses from cyberware and bioware are tracked separately". Then you take the larger one plus half the smaller, and you add those together, and that's your total Essence loss. "Any reductions to a character's Essence" -- from cyberware, bioware, heavy drug use, vampires, etc --"will also affect his Magic or Resonance" (p 84). Your Magic or Resonance goes down (not just the max, the actual attribute) by one for each point (or fraction of a point) of Essence you've lost.
The Sensitive System flaw means that Essence losses from cyberware
only are doubled. So if you get a cybernetic leg (Essence cost 1) it costs 2 Essence. If you also have a Digestive Expansion (essence 0.5), that still costs 0.5 Essence. Your total Essence loss would be 2 (cyberware) plus .25 (half of bioware), 2.25. And your Magic (or Resonance) would go down by three.

must ... kill ... all...humans