As mentioned in my post on the Chestburster mechanism, the two motors aren't mechanically linked, causing a problem where one motor would crawl ahead of the other. To play it safe, I devised a method of avoiding this problem. I reprinted a few of the parts and added 2mm steel rods to act as guides for the motor carriage.
The metal rods are attached to the ends of the whole mechanism.
The rods are threaded through wings on the carriage to keep it straight.
And here's a pic of the new assembly.