You got The Stygian Abyss in my Vengeance of the Kilrathi! No, you got Vengeance of the Kilrathi in my The Stygian Abyss! No, wait, you're both right!
An apparent sad end to the Sons of Kilrah isn't the first Ultima Underworld/Wing Commander, connection! The advent of the CD-ROM in the early 1990s made re-releasing formerly diskette-based games far more affordable. As a result, Origin developed a number of succesful promotions involving existing titles: offering older games to retailers at a discount, collecting mission disks and speech packs in 'Deluxe Editions'... and pairing seemingly-unrelated games to get players hooked on multiple franchises.
The first of these releases was a CD-ROM containing the original Wing Commander and Ultima VI... and it was later followed by a disc with Wing Commander II and Ultima Underworld! The hope was that the packs would appeal to bargain hunters (as you can see, the fact that this package is a "$159.90 value" is highlighted on the box cover) who may have been familiar with only one of the games. Then Ultima fans would try Wing Commander and get hooked and vice versa! We can only hope that GOG will over the same pack someday.