Perhaps if you bothered to read the Syl Arena link I posted, you will see there is a very simple explanation of how to put on flash in group A and another in Group B, they can then be set to ETTL and Manual separately!!
In fact, i did bother. I read both links from top to bottom, including the links Syl refers to in the text. I even printed it out. Also I watched the youtube video and two related videos from the same author.
For example, Syl does not show my how to turn a group off from the st-e3-rt, and that's pretty important for my workflow.
It seems like having one group on ETTL and one in manual is only possible if you use cameras past 2012, Im on 5D mkIII so I should be able to mix it up, but my flashes reset to M after exposure.