Did the Alliance and the Empire ever fight a common enemy outside of Shadows of the Empire?

Question

In the now-noncanonical novel Star Wars: Shadows of the Empire, a space battle breaks out. On one side, the Millennum Falcon (carrying Luke, Lando, Leia, Chewbacca, R2-D2, and C-3PO) and Rogue Squadron (led by Wedge Antilles); on the other, the crime syndicate "Black Sun". Before long, Black Sun forces are close to wiping out the Alliance forces facing them.

Suddenly, the Imperial fleet arrives, led by Darth Vader aboard his Star Destroyer. The Rebels realize they are doomed, and they are starting to say their goodbyes when a force of TIE fighters surge past their ships and attack the Black Sun fleet. As the battle unfolds, the TIE fighters occasionally send a blaster bolt or two towards an X-Wing, but it is clear that their primary target is the Black Sun fleet.

Black Sun was allied with the Empire, but its leader, Prince Xizor, had a personal vendetta against Vader, and attempted to hurt Vader by killing Luke. Vader discovered Xizor's plot, and ordered him to call it off, but Xizor ignored him. Vader intends to turn Luke to the Dark Side and overthrow the Emperor, so he is desperate to keep Luke alive. The battle ends with Vader destroying Xizor's space station, and presumably Xizor himself, who was last seen aboard the station.

Has a situation like this - with Alliance forces and Imperials finding themselves fighting a common enemy - ever occurred before or since?

Answer 1

In Legends there is The Truce at Bakura - a small rebel fleet answer a distress call from an Imperial governed world shortly after the Battle of Endor.

The rebels end up fighting alongside the Imperial garrison forces against the extra-galactic Sii-Ruuk.

Answer 2

If you accept displays of unity between the political successors of the Rebellion and the Empire, then the most notable instance would be between the New Republic and the Second Imperium against the Yuuzhan Vong. After the war against the Yuuzhan Vong, the two united to form the Galactic Alliance, and there are no direct, wholly separate, and parallel lines of succession from the Rebellion and Empire each thereafter.