If you use both Wunderlist and ToDoist and want the best kind of sync up then set up the Mail2Wunderlist feature and run this recipe till Microsoft allows Wunderlist syncing with IFTTT

This Trigger fires every time you create a new task in Todoist.

This Action will send an email to up to twenty recipients from your Gmail account.

Sync ToDoist with Wunderlist using Mail2Wunderlist feature