But, if I reduce the power of the AP..... will reduce the signal and quality of the other clients that has the signal and CCQ good, Am I right?
The only way to make all clients happy - or, rather, make the two close clients happy and keep the rest happy - is to increase pathloss of the two closest clients. Professional way of doing it would be to introduce a 20 dB atenuator between client's antenna and radio. Since we're living in WiFi world, trying to get by with low cost toys (which don't allow to easily place anything between antenna and radio), you can achieve something similar by turning client's SXT away from AP ... just observe reception on client and turn it in direction with least interference (you're trying to reduce signal level, not increase interference). It might be necessary to turn SXTs as much as 90°, SXT doesn't seem to have very narrow-beam antenna.