I just picked up the Lightheart Duo and its a pretty neat tent. Only had a chance to use it one night, but I like it. I had it fully guyed out with two of the four doors open. My wife and I slept in it with plenty of room, I'm 6'2" and broad shouldered and I did not rub against any walls. I did hit my hiking poles with my bag, so keep that in mind. Mine have a grippy area that grabs my bag.
We did have some condensation at our heads and feet. I think where the floor and tent come together, the air gets trapped there and condenses. I am going to try it with both doors open this weekend. If you are not too tall, it shouldn't get on your bag. So far, I am happy with it considering its 36 oz with stuff sack and stakes.
That being said, there are a number of steps down, in weight, that exist below your current tent. The Big Agnes Copper Spur UL2 is little under 3.5 lbs and would only be a reduction in interior space, compared to the half dome. Its still double wall, free standing. A next step down would be the Big Agnes Fly Creek UL2. This is another step down in interior volume - to the point of being pretty tight - and has a single door at one end. Thats around 2.8 lbs. Other manufacturers have similar steps if you look around.
If you still wish to go lighter, then you are looking at tarps, floorless shelters combined with bivy's or inner net tents. This can get you below 2 lbs.
Lots of options and it will probably take some experimentation to get to a final configuration.
On a side note - there are lots of UL tent options coming out this spring. Something to consider if you are not in a hurry.