Unless you want to make Blue/Custom/Purple gear there is no need for the secondary Crew Skill for any Crafting Skill.
Underworld Trading, Investigation, Diplomacy, Treasure Hunting are all not required to craft, all they do is allow you to make the Blue/Custom/Purple items out of your own resources instead of going to the market.
Hell technically you don't even need the primary gathering Crew Skill for your Crafting Skill if you are willing to be a slave to the market or have another character with it.
For example my Artificer doesn't have Archeology at all, because I have a Synthweaver with it.