If when you say profession you mean smithing and wood working for example. I think you can only learn on at a time, I THINK. This is how I think it works:

Say you start out by joining Smithing by joining the guild in which ever kingdom you’re in. You can get image support form those NPCs that are part of that guild when creating things.

But say later you want to make a potion so you join the Alchemist Guild; you no longer belong to the Smithing Guild, so I don't think you will get image support any more. Whether or not the skill will keep going up I don't know, now that you belong to different guild.

I hope Lionx or someone else will come and fill in the gaps or correct me if I am totally off base. I only tryed Smithing and Fishing in beta so this is only an educated guess.