Okay I looked it up, and I was sort of right.
You can't get Rath Rubies in the quest rewards of the lvl 5 Double Trouble quest, so the only chance we'd have of getting them there is the 2% one from tail carves. There is a lvl 6 version featuring tougher Rath twins that gives rubies, but obviously we can't do that one yet, so we are better off fighting Rathian and Rathalos individually.