I have no expertise in this, but I'm a consumer who did a lot of research before selecting a direct vent gas insert for an existing prefeb fireplace. Go to google and key in "see thru direct vent gas fireplace". Quite a few websites appear. The one by Monessen seems interesting to me.

I guess one of the questions you need to answer is whether any direct vent gas system can be installed in such a way that it will work in a see-thru chimney, or whether you need to go with a unit specifically designed for that purpose. I would call up some of the big, reputable direct vent manufacturers on their 800 customer service numbers. I think most of them will be able to answer your questions. Unfortunately, the manufacturer I went with, Jotul, is not very accomodating with their retail customers. If you call their 800 #, they direct you to their dealers.

I think it's a great idea to heat 2 rooms this way because these units really generate a lot of heat and can easily meet the needs of 2 rooms. Ours sometimes feels like a blast furnace, and we have to turn it down some. I also think it's prudent to plan the whole thing far in advance since you're building from scratch. I would ask around to try to indentify the most reliable installer, because these things are a little complicated and are sort of a specialty within a specialty. I have a feeling most builders and general contractors have little experience in direct vent units. It might be best to ask your builder to coordinate his chimney construction with a direct vent expert you identify separately. I think most builders subcontract the cheap, useless gas logs they usually install. So it shouldn't bend them out of shape for them to work with somebody you identify.

Good luck.