Be sure to shop at a store that sells knight swords. Have your character in a class that can equip knight swords, put the 2-swords option on. Put the shield in the right hand and the sword in the left hand (i.e. equip the items backwards from normal), then try best fit. The shop assumes the character wants to carry a sword in the right hand and puts it on in place of the shield. Agree to best fit, pay the 10 gil, and you're done!