With Unifi devices you can setup a minimal signal strength before it kicks clients off. In practice I haven't had to enable that hard cut-off yet. It so turns out that modern smartp homes such as recent Androids and iPhoneswill reconnect to a stronger AP. This was traditionally always considered as something that devices just don't do. Most still don't today but some the devices I have recently used (including some tablets like the Microsoft Surface) will reconnect. I did not need to enable features such asFast Roaming either. It seems to just work (haven't seen it work so smoothly on devices several years old).
If you mesh the two APs, you may need to experiment a bit to find the optimal location. Meshing happens over the 5 GHz radio (this is true for the Unifi AP-AC-M-Pro devices at least. I haven't checked if this is true for...