Two Field For The Price Of One Combo

I have a combo box which looks up vessel names, it stores the Vessel ID in a field [Vessel ID]. This all works fine, no problem, easy. However as part of the vessel details I also need to know the service the vessel is providing “transhipment” or “direct”. If it is a “transhipment” service I need to show a second Combo box with the on carriage vessels. That’s also easy, I just hide the second combo box and only show it depending on the service status of the vessel.

Now I figure I just need to have a field [Vessel Status] which is updated after the first combo box is updated and then depending on the value show or hide the additional combo box. The problem I’m having is updating the [Vessel Status] field once the first combo box has been update.