I must emphasise that what follows has no basis in practical experience - I have never run a twin screw boat.
If you imagine a propeller screwing itself through the water, the faster it turns the quicker it moves forwards (ignoring drag, slip and all the other annoying factors of real life). So for a given number of revs (1500 in your case), a particular pitch of propeller will give a certain speed. Now consider twin props of the same pitch. To get the same speed, they are going to have to screw themselves along in exactly the same way as a single prop. But as they share the work, each motor will draw less current (or need less power from whatever source it comes). So purely theoretically, your two props will need to turn at the same revs as a single prop to deliver the same speed. However, having the work shared means each prop will operate a little more efficiently, leading to an increase in speed for the same revs, or reduced revs to achieve the same speed. But exactly how much is beyond me I'm afraid.
Hope that helps a little.
Greg