Some speculation on the theory of mixing such compounds:
It seems to me like there is a minimal structural difference between JWH-122, JWH-210 and JWH-182 being the 4-methylnaphthalene, 4-ethylnaphthalene and 4-propylnaphthalenes respectively.
Meaning they are almost the same but got a 1 carbon, 2 carbon and 3 carbon chain somewhere. Sometimes such a difference can be like night and day but at a lot of other times the change is just subtle.
The difference may be in potency, some of them being slightly stronger/weaker than others, or shorter/longer lasting. But apparently they are classified as analgesic, i.e. painkilling cannabinoids, I have no idea how stoning or psychedelic they would be.
Also no idea what the advantage would be of combining them but generally combinations have a factor of unpredictability that adds up, it can become harder to determine how high the dose has to be relative to each other. On the other hand, these cannabinoids tend to play well with each other, and you just have to find a new sweet spot with a mix. And because they seem to be so alike, the unpredictability risks would likely be just as minimal as the advantage.
There is one general advantage I can think of with synthetic cannabinoids and that is that ones that are quite different from each other can be combined to enrich each other, making it seem more like the broad spectrum palet you get from natural cannabis products.
In conclusion I don't think there is much use to combine them but also not a big risk to be expected. I can imagine how vendors would like to apply mixtures to herbal blends for the aforementioned reason that they can provide a wider array of cannabis-like effects.
That would also depend on whether you want most of all to emulate a generic cannabis high, or isolate a small part of the effects. But again: none of that seems to apply when the synthetics are very similar to begin with.