If we are going to use the "blue car" and the "car blue" argument, than SS/RS would be correct.
If we stay with that argument, than we go from biggest to smallest. IE: GM Chevrolet Camaro SS/RS. GM (manufacturer), Chevrolet (division), Camaro (model), etc. and so on, biggest to smallest.
The bigger feature of the two is an SS. The smaller feature is RS. (It also cost more money for the SS option than the RS option). Also, Chevrolet themselves help answer this question as the SS emblems take precedent over the RS emblems when both options are ordered. Therefore, they thought it more important to emphasize the SS over the RS. Thus, SS should come first.
They had embems for each option separately, but no emblem to represent both together. Maybe it's because GM didn't know the answer to this question either and therefore, just picked SS to take over.
I like SS/RS because it breaks up the S's. If you take out the slash mark, you have either SSRS or RSSS. To me, SSRS looks better than something that sounds like "ARSE". Since I label a lot of picks that both options and often leave out the slash mark when saving a name for the jpg, it just seems to look better with SS first followed by RS.
But then again, either way works and gets the message across. Thanks a lot for making me think about this oh so trivial yet gosh darn important matter.