Don't worry, I've only ever seen the older Zorua disguise itself as the newer one while SOSing. I've shiny hunted Zorua using SOS encounters twice, and the shiny appears shiny when it first appears. In theory, if a shiny called a non-shiny, then they would both appear normal, but by that point you would already know that one of them is shiny, so even if you forget which side it was on, just False Swipe them to remove the disguise and you should be fine.