The All in two (AI2) nappies are similar to the AIO when assembled, however, the absorbent inserts are not sewn in, instead, they are attached to the cover with a snap or velcro. The inserts are then detached once used and are washed/dried separately. This decreases overall drying time and allows for more flexibility. Inserts can be taken out to decrease absorbency and bulk (when the baby is very small for example), or additional ones can be snapped in to accommodate a growing baby. While AI2 nappies do require some assembly, this system is generally one of the quickest to prep.
The AI2 covers can also be reused multiple times. It's possible to change the insert, attach a clean one, and reuse the nappy again. It's generally recommended to have at least 2 covers and interchange them, so one can air out before being reused. This makes the AI2 system very economical since additional inserts are substantially cheaper than the full nappy.
PLEASE NOTE: Read product descriptions carefully as not all AI2 covers can be reused in a similar way.