There are many, many other issues that need fixing. The 2nd driver problem is so far down my list that it does not even register.
I am sure that everyone else who has posted a bug in the bug reporting web site that is still waiting for that bug to be fixed (and some go further back than Trainz 2010) would rate their issues much higher than the 2nd driver issue.
Does the fact that the second driver is missing cause any operational problems for TANE/TRS19?
It is all a matter of priorities. Minor cosmetic issues, such as a 2nd driver missing from the cab no matter how quick it would be to fix (so you claim), should not take priority over issues that affect gameplay.
All will be fixed (or rendered unnecessary) in time.