Split current load through different wire
Actually current will tend to self balance across wires. The one taking the bulk will heat up and gain resistance so the others will become more desirable. 

However, length is critical, and voltage drop along 28AWG will be high for any significant distance. 

In this equation you also need to back off your math by at least a factor of two. YOU DO NOT want to run at the rated limit for 28AWG or it will overheat.

Not sure why you need to use ribbon cable for this application, but there are alternative solutions available that will connect to a standard .100 ribbon cable header that allow you to use heavier and more suitable wire types using crimped pins.