If I may.
Both impellers on both supercharger stages were on the same shaft on 2-stage Merlin (and Griffon, as well as on many German 2-stage supercharged engines). When driving gear was in 1st speed, both impellers were rotating at same angular speed. Or, doing the same RPM if you will. For example, on the Packard Merlin V-1650-3, the impellers were rotating 6.391 times the crankshaft RPM - for 3000 rpm the crankshaft will be making, impellers were making 19173 rpm, providing the boost of 61 in Hg at the rated (low) altitude.
When driving gear was switched up to 2nd gear, again both impellers were rotating at same angular speed (or, on same RPM), this time the ratio was 8.095:1 vs. crankshaft - for 3000 rpm the crankshaft is making, impellers will be making 24285 rpm, again providing the boost of 61 in Hg at the (now much higher) rated altitude.
The faster the impellers rotate, the more compressing work they will do, but also suck up more power (we're talking about gear-driven supercharges, as used on ww2 aero engines,but not just on them).
Low impeller speed (= S/C drive is in low gear; British name is 'MS' - medium supercharge) means the S/C will be using less power to provide required boost, so there is more power left to the prop. It is okay, since at altitudes the low gear is used has thick air around, so there is less of need for the S/C to compress the air. High impeller speed (= S/C drive is in high gear; British name is 'FS' - full supercharge) means the S/C will be using more power, but again that's okay since it will be providing the required boost at high altitudes. More boost = more power; the trade-off favors the 2-speed superchargers.
Some engines were outfitted with 3-speed 2-stage superchargers - yes, the drive gear is outfitted with 3 sets of gears. Such engines were the Jumo 213E and F (late ww2), as well as RR Griffon 100 series (post war)
Widely produced were the 2-speed 1-stage supercharged engines - 2 sets of gears, one impeller.