Set 3368 Space Centre missing one 4211805
This looks like an oversight in the inventory printed in the instructions. Your interpretation of the instruction step requiring two of these is certainly correct.

The Bricklink inventory shows 2 of these, so that's what would have been included in the box. The replacement parts page on LEGO.com also indicates that this set should include 2 of the 7M axles.

It's likely that the seller just checked against the printed inventory and didn't include the second axle due to the misprint.