Dallas & Brett were NOT both being transformed into alien eggs, this was only happening to Brett while Dallas was the intended host of the face-hugger. For clarification, here's how to find out more in the U.K. version's Bonus Disk (U.S. version probably differs) from the Alien Quadrilogy Boxset.
Look up Alien_154>A1 in the Main Menu, goto Laser Disk Archive_445>LDA then Table of Contents.
Scroll through the contents to Part III: Post Production/Chapter 17: Editing and Music. This section contains pages from a booklet for the laser disk version, featuring various pictures and quotes about the films post production, including several deleted scenes it calls out-takes.
Continue through these (NOTE: it may take a while as you have to start each out-take).
Go past several production stills, then an interview with Scott about why the Cocoon scene was removed from the film, after which there are 4 more production stills relating to the cocoon scene.
Immediately after the 4th still is a page reading:
Since the concept of the sequence was that brett was being transformed into an egg containing a new face-hugger, with dallas as the intended host, one of Gigar's finished eggs was used to complete the construction as filmed.
After that is another still of Brett in the cocoon then the uncut sequence itself.