I play all my characters fully as equals, not as a main plus alts, and they all advance in crafting. So far as motifs and recipes etc are concerned, if the characters that find them can use them then they do, otherwise they go into the bank for another character to use, or they're mailed to a character on my other account if they're needed there.
Like most people it seems: 1 Master Crafter, 9 traits and all motifs i can find go to him (it's a endless job to have all motifs).
3 other max level crafters with all base racial motifs (including Imperial), and events motifs and some traits (work in progress). For writs.
3 other have at least 1 crafting skill line fully leveled - other than provisioning.
I have 14 characters and 14 crafters. All but the Warden are maxed on everything, but only 2 have been keeping their motifs up. The Warden still needs some Enchanting.
Even if I wasn't maxing out any other skill, I'd get enough alchemy to get the Medicinal Use 3 and enough Provisioning to get Gourmand 3 and/or Connoisseur 3. There's really no excuse not to.
My main is a Sorcerer and will be doing the majority of crafting, including traits on things he won't need like swords or bows. My other two will still gather and deconstruct so they will have passives that increase material gathering and extraction. Some points on the alts will go towards gathering better materials but there's no real reason for them to learn provisioning or alchemy.