An interesting thing I have learned: it may be impossible to obtain certain wild shinies on certain save files of Emerald WITHOUT using a synchronizer.
I've been trying to RNG a shiny trapinch and just succeeded, but not without first catching a shiny sandshrew that appeared on the same frame during a previous attempt. So it is indeed possible to catch different species of shiny on the same frame.
However, I also know for a fact that RNG sets the species and held item on each frame; these are locked in the same way IVs are. While looking for my shiny trapinch, I encountered, on two consecutive attempts, a pokemon with the same IVS and gender. So I know I hit the same frame twice in a row. However, both pokemon were cacnea, which has only a 6% encounter rate in Emerald, and each was holding a poison barb, which only 5% of cacnea do. The chances of encountering this twice on the same frame by "chance" is extremely low. It's also worth noting that the cacnea encounters meant my synch hadn't worked that turn.
So my hypothesis is this: the reason you can get different species of shiny on the same frame when species is locked for each frame is because synchronizers do not skip entirely to a new frame. I think what is happening is that when a frame is selected upon entrance to battle, the RNG first checks species before anything else. THEN it checks whether synchronize works. If no, the rest of the frame loads as predicted. If yes, the game keeps the species that the rng already called for, and skips to the next correctly-natured frame before loading the rest of that frame to determine IVs, gender, and shininess. In this way, using synchronize can create several pokemon species with the same stats, gender, shininess, and everything else. Shiny pokemon caught in the wild by using synchronize are actually hybrids of two different frames.
This means if you don't use a synchronizer, and frame 2938 is a shiny zigzagoon on a certain route, it will always be a zigzagoon unless you choose a different frame (which may have worse IVs or an undesirable nature).
It also means that the frames between synch frames are actually important, because you have to hit one with the species you want. If you've only got five frames between synch frames and none of them produce a skarmory, you will never get a skarmory, you will get a lot of spindas, and you have to a choose a different frame.
This is probably not relevant to a whole lot of people because wild shinies in Emerald can be obtained with better IVS in other games much more easily, but I still like playing Emerald and I thought I'd share my findings.
