I added a second one, and things got substantially worse. Devices would stick to the faraway access point because they could still just see it.

I tried things like "roaming assistant", to forcibly disconnect clients below a certain signal strength, but that interrupted ongoing WiFi phone calls and such.

It's possible that 802.11k and r help, but at the time I investigated, many clients didn't handle that properly.

Instead, I mounted one access point on the ceiling in the middle of my apartment, and WiFi is much better.