Welcome to the ODK forum, @oichirobert ! We're glad you're here. When you get a chance, please introduce yourself here. I'd also encourage you to add a real picture as your avatar because it helps build community!
It sounds like ODK-X should provide the functionality you are asking about. I think we will need a more detailed question in order to provide a more exact answer. ODK-X does provide "Bi-directional synchronization of data across devices." You can read more in the documentation linked by @mathieubossaert above.