Question: I've extended the wifi in my home with a second wifi access point (AP) router, *wired* to the main one. The two routers are configured with the same encryption settings, same subnet, and same dhcp pool.

Their antennas are on non-overlapping channels, and they are configured with *separate* SSIDs (e.g. HOME-24G-*ext* vs HOME-24G). Would reconfiguring the extension router to use the main's SSID affect anything w.r.t. the AP-roaming decisions of modern phones and computers (in 2018)?

With separate SSIDs, I will sometimes witness wifi clients hanging on to a weaker signal on the main, and *not* automatically switch to the other extension SSID (or vice versa). So, I'm wondering if uniting the SSIDs would facilitate transitions to the AP with a slightly stronger signal.

*Note 1:* This is related to this question on [same vs. different SSIDs](https://superuser.com/questions/1215392/dual-band-wifi-router-to-dual-band-extender-can-should-i-use-a-single-ssid-name), but not quite the same. My two routers are wired, so I don't think that the issue where *"clients hang on to a stronger repeater signal when better performance would be achieved on the main router (despite a weaker signal there)"* applies.

*Note 2:* WDS wasn't an option -- The main router doesn't support it.

*Note 3:* My setup: the extension router is wired from a LAN-port to a LAN-port of the main router. And dhcp is handled only by the main router. This is called "[Bridged AP](https://openwrt.org/docs/guide-user/network/wifi/bridgedap)" in OpenWrt parlance.

Answer: 
Wi-Fi clients generally have a "roaming threshold", which is a signal strength (RSSI) value below which the client will consider its current AP to be too far away, and start doing scans to find a better AP to roam to. If it finds any other APs publishing the same SSID, the alternative AP with the highest RSSI must be enough better than the current AP's RSSI to be worth roaming to (to avoid thrashing/flapping back and forth between two equally bad APs).

Scanning is expensive, especially if you have to look for hidden SSIDs or scan DFS channels, so some clients may choose to do quickie roam scans that are less likely to find APs publishing hidden SSIDs or APs on DFS channels. So for best results, be sure to broadcast your SSID, and put your APs on non-DFS channels.
If there is two or more Access Points with the same SSID, the Access Point with the stronger signal is used. The value that determines the "strength" is more or less the RSSI. The reason for this is to prevent an easy "Evil Twin" attack (although, someone with a decent Wi-Fi adapter can still pull this off) and so devices have the best connection strength possible. Schools, universities and any place with a big area for a Wi-Fi signal to cover, most of them utilize this function.