I have one, fitted to a Tubus rack. Built like a battleship. Filled it to the max with cans of beer several times, even with the flap extended to the max, no issues with stability. Used daily on my 20km commute all yr round. Exposure Strada front light, Bontrager Flare r on back.
It all depends on how big your bike is and how high the rack is fitter. I am 5, 7” and ride medium sized bikes. It is not possible to do that on any of my bikes. The rack for the saddle bag clashes with the pannier rack, it is one or the other. You have a few options though. You could use a smaller saddle bag without a rack. You could get a bag to sit across both panniers or you could get the rack top pat like the Super C rack top bag that sits long ways in the space between the two panniers.
Do you know if the Carradice Carradry handlebar bag will fit on road bike handlebars?? I've only seen it on mountain bike handlebars and am wondering if it is to big for roadbike handlebars??
The bag handle bar bag, not the rack bag in this video is 26cm wide. I have used them on drop bars. Measure between your handle bars but also make sure you have room to change gear if you have brifters (combined gear/brakes).