In my recent herbicide test applied September 23rd, 2013. On clover, oxalis and dandelion: Speedzone was best, but Gordons T-Zone and Surge were almost as good. A three way was only fair.
However...nothing seemed to have any effect on wild violets. Even various combinations of the above products were not effective. Fall must be the wrong time of year. Spring is better.
I made combinations by spraying the first product then applying the second on top of the first. Mixing them together in the same sprayer would be about the same.
When I mix two products together for tough weeds--I mix both at the label rate per thousand sqft. It is essentially then double strength. Remember there is a limit on 2,4-D-- I think it is no more than 2 lbs per acre per year. Don't exceed the 2,4-D limit and keep it low so you can apply 2,4-D at another time during the year.