Depending on the cause, dry mouth/dry tongue can be either separate or related. You have several different saliva glands in different locations of the mouth. Dry mouth/tongue can be caused by anything from simple reactions, to medications, to a systemic issue. With a little more information I might be able to give you a better answer.
How long has your tongue felt dry?
How much caffiene do you have in a day?
Are you on any medications?
Do you have an medical conditions?
My sister constantly has a dry tongue and cannot speak if she does not moisten it with something