Usually, one contact can only participate in one sequence at a time within the same Sprouts account. However, there is an exception:
Team Edition with the Public mode
💡If your account is a part of the team with the Public mode enabled, you may add a contact to multiple campaigns simultaneously. These sequences must belong to different team members but it is possible.
The main reason you cannot add the same person to multiple sequences is safety. To ensure the contact does not get different emails from various campaigns on the same day, Sprouts only lets you have one campaign per contact per account.