I'm not so sure why you would want to, but if you're going to do it, the two pumps would have to feed into a "T" and the outlet of the "T" would have to be 1.5 inch tube in order to handle the flow. That would look quite bad, if your attempt is to make the tank look better with only one outflow pipe.
Not really worth the effort and two pumps will give you good cross flow which is really what a good tank needs.
$.02