You can use more than one MI online casino promo code, but it can’t be for the same type of bonus. For example at BetMGM, you can only take one welcome deal at most gambling sites, so you can only use one welcome deal BetMGM promo code.
Of course, if you want to join two different sites, you could claim a welcome deal at both sites, and after the welcome offer, you can use promo codes for all kinds of gambling bonuses, but you can only use a code once.