Adam Saxton has a quick video demonstrating a dual KPI custom visual:
The Dual KPI efficiently visualizes two measures over time. It shows their trend based on a joint timeline, while absolute values may use different scales, for example Profit and Market share or Sales and Profit.
Each KPI can be visualized as line chart or area chart. The visual has dynamic behavior and can show historical value and the change from the latest value when you hover over it. It also has small icons and labels to convey KPI definitions and alerts about data freshness.
I looks cool, but I dunno; my philosophy is that man cannot serve two KPIs.