Two-way valves,

Two-way valve components allow flow in a device in two directions but at a certain pressure differential. The pressure differential can be the same in both directions but also different. For example at a low (P in one way and a high (P in the opposite way. The flow can go either through the same or through separate conduits. An example of an application is for example a combined vent and relief valve that allows air to be drawn into a container when there is vacuum and that allows air to be vented from the container when there is overpressure. You can always use two one-way valves to realize that function but the preferred method is to use one component that combines the two functions

Click below to find the component that combines both

Umbrella/Duckbill Combination valves
Dome valves