The effects of more than one Magus of the Chains are cumulative. If there are two of these on the battlefield, each of them will modify each draw (after the first one if during the draw step) and will cause the player to discard or to mill a card. As they resolve in order, the player must discard if possible. Once the player fails to discard and instead mills a card, all further effects of each additional Magus of the Chains will not do anything. This is because the mill also replaces the draw effect and the player is no longer drawing a card. You handle them in order. Each one makes you discard first and then continue or else mill a card and lose the draw.