There are two exceptions to this: Explorer and Woodsman. These two vocations have only one producing profession and two gathering professions (all other vocations have two producing professions and only one gathering profession). So for Explorer and Woodsman you can work on both gathering professions at the same time.
Originally Posted by TinDragon
For Explorer you have both Forester (feeds Tailor) and Prospector as the gathering professions. Work on both of these together. Otherwise you will have to go back to lower level areas or buy the ores on AH. By working on them together you can be selling the excess ores on AH. Besides ores, Prospector also gets you gems (for Jeweller) and special ingredients (for Scholar).
For Woodsman you have both Forester (feeds Woodworker) and Farmer as the gathering professions. Farmer can be worked on at anytime as you only need to be at a crafting facility (farm field). Besides crops (for Cook) you also get special ingredients (for Scholar).
To clarify what TinDragon said, the crafting guilds are for the producing professions only and not the gathering professions. So choosing the crafting guild for the producing profession that is associated with the gathering profession will make it easier since the character will be self-sufficient.
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