You need to check the local building codes regarding venting more than one device into a single flue. It is very tricky, as heatseeker says, and you'll want to follow the code so your insurance company can't deny any claim should there be an accident.
One thought is consider a fireplace insert for heat. It's simpler to do and could be a more cost effective way to go. Also consider a coal burning insert, if coal is available to you.