You can have stained concrete floors on a second story of a house, even if it has wood subfloors. The process consists of laying building paper, nailing metal lath, filling the metal lath with a repair mortar, apply modified-polymer overlay, acid staining, and sealing.
The modified-polymer overlay will actually bend and give some without cracking; since the cement is mixed with a polymer, which is like a liquid rubber. The weight of the total process is not anymore than laying tile on the second floor. The total thickness is about 1/4 of an inch.
Below are photos of the process taken to complete this job.