I don't see the benefit of using both, but I can think of negatives. Let me explain.
The amount of bacteria that grow will not be enhanced by the additional biological filtration. The system is only capable of sustaining a certain bacterial population, depending on the amount of organic waste and acids which are introduced in the form of fish waste and food decay. A sponge filter alone, or a properly maintained undergravel filter alone, should provide you with adequate surface area to house these bacteria.
On the flip side, using both will double your maintainance requirements, because you will have to service both units. Not to mention the additional cost and wear on your pumps.
I can't recall the exact year, but I will guess it was 1993. My fishroom at the time only had 8 or 10 aquariums. I finally made the move completely. Which is to say that i disconnected every undergravel filter all on the same day. Each aquarium was running either a sponge filter or a Penguin biowheel in additional to the u/g. I finally got up the nerve, disconnected the u/g filters, and it was one of the best decisions i have ever made in my fishkeeping years. I have never gone back. Sponges are cheaper, easier to maintain, and more flexible when the time comes to upgrade aquarium size.