An all-in-two is similar to an all-in-one in that all of the components are meant to be connect; i.e. the cover and absorbent layer are attached. The difference is the the cover and absorbent layer(s) is attached via a snap or velcro. This means that the inserts can be removed for more thorough washing and drying.
This system is appealing because like all-in-one’s it’s very similar to the experience of disposables. A bit less fuss.