Last year I decided to give a new life to my oldest guitar - a Washburn with a very pleasant/playable neck. I changed all hardware - pretty much using it as a guinei-pig guitar for testing new hardware. It ended up a brilliantly sounding guitar with Schaller loking tuners, Babicz bridge, Seymour Duncan pickups.
Unfortunately I thought about using stacked pots - and stacked potiknobs. PROS: - sturdy construction, looks good CONS - they did not match the only 250/500 pot found in store (from AllParts), had a hard time tightening them - the two pots (the ring and the knob) hardly work simultaneousely without influencing each other, ie: I cannot change volume without affecting tone and vice-versa
Bad choice from my part, I wanted to avoid drilling a new hole, now it seems a much better solution
The only thing I have to criticize is some kind of dullness the black poti knobs have, when first unpacked. I bought a lot of those knobs (single and tandem) and got the same strange "patina". Using a microfleece to clean them up helps, though. I've got lot of bass guitars and into three of them, I installed Glockenklang 3-band electronics. Since I don't like too many poti knobs, I'm using the stacked ones. Exactly for that purpose, these tandem knobs work absolutely fine. On my Warwick $$5, you really can't say if the bass didn't have that tandem-knob right from the start ;-) Mounting is no problem, all you need is a small allan wrench (older models need a normal screw driver) and you are set. Pro tip: Use a sheet of paper, fold it 3-4 times and lay it on your bass before you mount the knobs, touching the poti axis. This way you ensure there's a minimal gap between knob and body to leave it moving freely.