In AutoCorrect I have added eqmz in the left-hand box and ⇌ in the right-hand box. This means that every time I need an equilibrium symbol I just type eqmz.
See here for how to add to the AutoCorrect table.
See here for how to find the Unicode symbol for the equilibrium arrow on the keyboard.
To find the Unicode symbol of any character use my macro here.
Chemistry Word VBA macros 