Series: Gemini (Interconnected Standalones) by Penelope Ward

Each book is a standalone (except for the Jake books), but the characters crossover.  Books are listed below in order of publication date.

Gemini  Jake Undone  My Skylar  Jake Understood  

Gemini (Allison & Cedric)
Review
Buy: Amazon | iBooks | B&N | Kobo | Paperback | Audible

Jake Undone (Jake, #1) (Jake & Nina)
Review
Buy: Amazon | iBooks | B&N | Kobo | Paperback | Audible

My Skylar (Skylar & Mitch)
Buy: Amazon | iBooks | B&N | Kobo | Paperback | Audible

Jake Understood (Jake, #2) (Jake & Nina)
Review
Buy: Amazon | iBooks | B&N | Kobo | Paperback | Audible