I would do it sometimes, but not always. I think the confusion principle applies well here. Sometimes to mix it up with the legs do the imax a day before or after weights - maybe a couple of weeks out of a one month rotation. Though, if you find it works well for you and you arent' unduly stressed or stiff, keep doing it. If your legs feel tired or stiff and the back-to-back does not seem to be agreeing with them, or your plateau or lose strength or endurance, don't do it for a while. Everyone is different and responds to different things, so I think experimentation is needed to see how it works for you.