Two-way humidifiers will emit and absorb humidity while seeking equilibrium at a specific humidity level, such as 70%. The first two-way humidifiers were often referred to as reverse osmosis (RO). 'Osmosis' means the diffusion of water through a membrane (evaporation), and 'reverse' refers to the inhibition of that process.
RO humidifiers units were popularized in the 1980s and 1990s. The first generation used a generic green foam as the hygroscopic medium: these RO units used an organic chemical called propylene glycol (PG) mixed in a 50/50 ratio with water to achieve 70% rh. Today, most two-way humidifiers still use PG to inhibit over humidification, yet they can use different hygroscopic mediums, such as crystals or plastics, instead of green foam.
RO humidifiers which use PG require a near perfect 50% balance with water to ensure proper humidity - too much PG can clog a hygroscopic medium like foam which will result in the humidifier not accepting water. Too little PG can lead to over humidification. Often, the key to successfully using an RO humidifier is to only add PG if your humidor is too humid. Another solution is consistently refilling the RO humidifier with 90% water and 10% PG (90/10). This ratio will allow the PG to break down (into inert sugars and water) instead of accumulating and potentially clogging the humidifier's hygroscopic medium.
Other two-way humidifiers can use salt-based solutions which release and absorb water until the ambient humidity reaches a specific level. An example of this principle is table salt, which will stop emitting moisture when the ambient air is 75% rh.