I have a double fireplace one opening is in the living room the other in the den . The fireplace in the den appears to be separating from the wall - Any thoughts as to why this would be?

Houses rack, shift, settle. So do fireplaces. Depends a lot on age, materials, foundation type, soil type, and how the whole house was constructed. You don't even say what type of fireplace it is, brick from base of basement to roof, or firebox, or what. Call 2 contractors to come look.

On a slab? On a crawl or basement?