Hi all!
I primarily play a bear tank but I’m not my guilds tank for raids so, in some instances, Ill be a balance druid. Simply because I like being a caster if Im not a tank. Is this viable or is the difference in gear between the two so bad I just shouldnt? Im considering being primarily a balance druid next xpac anyway with the introduction of delves likely being my new personal end game and raiding on occasion with my crew. What do you think?
Mastery haste and versatility are good stats for both spec.
So only the weapon has to be swapped
Different trinkets in addition to weapon and you’d probably be fine 
I think it’s pretty common to play multiple specs for a class, I know I almost always end up doing this. As mentioned by others you’ll definitely need different weapons and usually trinkets.
Beyond that, unless you’re running very high end content, you are fine sharing the other gear across specs. Your secondaries might not be perfect, you might not have perfect embellishments/enchants, but it’s not going to impact you below the top 1%.