No question is a stupid one. The question that you don;t ask and causes a catastrophy is the stupid one. LOL!
To answer your question, yes, it will work. Float valves work on bouancy. That being said, it will float in the middle of a storage tank. What will be a little tricky is the fact that, when the barrel is empty, you have both water sources dumping in to the barrel at one time. It is possible to calibrate your system so that it will give you the same mix every time, but it will take some tinkering. I would put the tap float valve on the bottom. RO will fill slower than tap so it should be on top. Let the drum fill with tap first then top off with the RO over time.
I would recommend shutoff valves just before the floats so that you can shut them off when you are draining the drum during water changes. When you are done with the water change, turn them on an let the drum fill. Another thing to take in to consideration is the fact that you MUST drain the barrell 100% at each water change to maintain consistent storage water parameters or, over time, your water will become more diluted with RO.
Hope that helps!