Surprise Duet (歌姫と共に Together with Songstress in the Japanese version) is Paralogue 3 in Fire Emblem Fates.
This paralogue will be unlocked once Azura has achieved an S-Support with someone.
The script for this chapter can be found here.
The player starts off in the middle ship. The enemy has half of their strength on two ships, which are placed north and south of the main ship. The boss is on the northern ship with several Berserkers while the southern ship is filled with Sorcerers instead. There are three Adventurers on each ship (one in the north serves as the boss).
Upon battle initiation, the northern ship will lock onto yours. Note that there are only a few tiles that ground units can move from one ship to another; flying units are not directly affected by this. At the next turn, the northern boat will depart while the southern boat will lock on, and vice versa for the remaining turns. The southern boat can be dealt with fliers, but because of the three Adventurers on the boat, it is recommended to bring other high-resistance units instead.
After a few turns, the top and bottom ships alternate in spawning pairs of enemies (usually a combination of berserker/fighter and mage/sorcerer). This is a great opportunity to train units, particularly when playing Conquest, where experience is usually limited. The boss does not leave his ship unless one of the player's units are moved within his attacking range; keeping them at a safe distance ensures that he does not move (and by extension the level not ending) until the last extra bit of experience is earned.
If the player does not need any experience, has already gathered enough, or simply wants to finish the paralogue fast, they shall grab a flier (paired up with a strong unit) and have them take out the enemy commander.