They would cancel eachother out, when 1st is shown, 2nd will be removed and vice-versa. The new message overwrites the old if they are on the same hud object.
You could create 2 separate hud objects and show both of them at the same time(assuming you're not using the same x,y coordinates). Just keep in mind that there's only 4 hud channels available, and there are other plugins that use those channels too and send hud messages...