It all started with the (amazing) SE Pair... 

Or 2 Smoke Erasers in rotation where the addition of the 2nd creates exponential lifespan increases for both. This was actually discovered accidentally and seen initially as an anomaly due to the remarkable amount of time each unit lasted. It was then introduced as a solution for heavy users to avoid restriction with single units. Then known as the way for anyone to crank up lifespan. It's now achieved 2 ways; via rotation of 2, or the addition of a Dock.

So breath moisture is the reason for short lifespans in competing units (300-500 uses is premature failure). Smoke Eraser easily doubles this with a single unit. Those wanting even more can add a 2nd for the extra downtime, or a Docking Station to dry over-wetted HEPA filters. Either method can be effectively used to increase exhales beyond 1500 as long as resin is being purged as needed from prefilter; handled simply with a soak in alcohol.

But since the introduction of the Docking Station, typical orders from Smoke Eraser vets have continued to be SE Pairs (or more), with most eventually purchasing a 2SE/Dock (once) to score the Dock for cheap. Smart since keeping a Dock on deck then minimizes the pairs they buy again while keeping a tremendous # of them wide open. The combo also tends to milk most exhales per unit. But that's just one example. Here's the bottom line. Whether needing to accommodate an excessive amount of breath moisture (heavy demand), or you simply want more bang for your buck, rotate 2 units or add a Docking Station. Simple as that. You'll be happy. Trust me.

As for the options below, although strongly recommended for heavy users, they're just as ideal for those seeking increased exhales or a maximization of Smoke Eraser value...

SE Pair
SE/Dock
2SE/Dock