The Parker Sisters series has two main characters: Melanie Parker and Brittany Beresford (née Parker).
Each book is narrated by one of them (with the other as a supporting character) and is a complete story that can be read as a standalone. However, there are ongoing storylines and character crossovers, and later books refer to events in earlier ones.
#0 Wedding Hells (short story)
Books 3 and 4 are in progress.