What is current capacity of IDC connected of 7 stranded 36AWG wire (listed as 28AWG)?
Based on this page which is super nicely detailed: http://www.mosaic-industries.com/embedded-systems/electronic-instrument-design-new-product-development/cables/ribbon-cable-current-rating
Ribbon cable is closer to chassis wiring (Free to air wires) than it is power transmission wiring (Closely bundled wires, some not exposed to outside air). A single 28 AWG (or in your case 36/7 stranded) wire can conservatively handle 1.4A by itself. 500 mA is much lower. 3M also finds 1.5 Amps as a max for their ribbon cable, as a derated to 30 Degree over ambient temperature rise. The absolute max is closer to 2.5 Amps!
If you dedicate 2 wires, (4 if you include ground), the temperature rise over each and the voltage drop over both will decrease, making it a better solution. But you can use a single wire if you wanted to.