I tried this, and I can say that they both over do it and spoil the capture!!!
1) Switch Off the Steadyshot & switch on the IS on the canon lens(tried with 24-105)
IS works very well
2) Switch ON Steadyshot & switch Off the IS on the lens(tried with 24-105)
IS works very well
In these two cases, I get almost 2 to 2.5 stops, they both are almost same!!!
3) Swith ON the steady shot & switch ON the IS(tried with 24-105)
IS doesn't work, upon that the focus takes a toll....blur blur blur!!!
I heard that IS & Steadyshot work well together on metabones, but metabones doesn't work well with few canon lenses the AF-S is slow!!
viltrox II works pretty well with 24-105 F4, 16-35 F4, and 400 F5.6L...I have these lenses and I can say that it they work very good, almost native speed!!!