I tried troubleshooting this by switching the connectors and raisers to each card and also switching their positions on the mobo but each of them in any combination individually gets the max rate but when both together one drops hash rates from 30.8 to 22!
isolate one by one, First use the riser with low hashrate on main PCI-e, install one GPU, if it has no prob, install the second GPU if still drops hashrate, check if OC settings has been took effect to both GPU's, I am not sure with overdrive tool, but in MSI AB it has an option that a separate settings could be applied to each GPU even its identical.