My helix has 3 1/2 turns and it only gains 2 feet. At the minimum and recommended 7" per turn, you would need 5 1/2 turns to gain 3 feet.
You actually don't need 2 helixes because that would use up almost all your space. Just one at the end of the long wall can serve as the transition between levels. Each level would be like a shelf that wraps around the room. At the "end" of each level (furthest away from the helix), you can create a loop, or a reversing loop that sends the train back along the same level.
If I get a chance I'll make a rough sketch to show you what I mean. Interesting project!!
