Combination writing instrument

A writing instrument, which has an inner and outer writing element with separate writing medium reservoirs. The writing elements are coaxially mounted in an outer barrel and axially moveable with respect to one another. A driving mechanism may be actuated to allow a user to selectively extend one writing element. The inner and outer writing elements may have an inner sleeve therebetween to improve, for instance, rigidity of the outer writing element and to prevent contamination of the writing medium reservoirs. An outer sleeve may also be provided over the outer writing element. The outer sleeve can function to hold various components of the outer writing element together, and to allow for clean and easy refill of the outer writing element, especially when the outer writing element is a porous nib with a filler-type writing medium reservoir. The writing instrument is designed so that a user can replace the inner and/or outer writing elements when their respective writing medium have been expended.