Not sure why you would want to do this in the first place, if such a thing did exist it would be very uneconomical. Wood burns at a much higher temperature than gas. From the numbers i remember at trade school (although its been a while) fireplaces are 20-25% efficient, meaning that every $1 you spend on gas for the thing, $0.80 goes up the chimney. And with the lower temperature you get from burning gas, im guessing that you would have to burn alot of it in order to heat this room you have there. The new gas only fire places are designed to project heat into the room a bit better then a conventional wood fire place (ie smaller flu, fans inside the fireplace housing that circulate warm air into the room, ect).
To have a dual fireplace would mean that your flue would have to be very large (too large for nat gas) and you would need a fan that could withstand immense temperatures from burning wood.
Believe the guy when he says that no such beast exists
hope this helps