As pmb00cs says, if you give them the same SSID+password you are
merging them into one network. Giving them the same channel is not generally an advantage unless there is a distinct gap at that channel, and even then may limit the wireless bandwidth.
If you have some channel scanning software (e.g. InSSIDer on Windows or Wi-Fi Analyser on Android) you can investigate. If they are mostly on 1,6,11 stick to that. If they are mostly on 1,5,9,13 (less likely) follow that instead.
At each AP location, if all of 1,6,11 are occupied by neighbours, choose the weakest and/or least occupied channel of those three.
When you give 2 or more access points the same SSID, you are telling your client devices that it is one network that they can join via either AP and roam as needed. How long they hold a weaker connection before roaming to the stronger is down to the drivers on each client device.
Giving them different SSIDs usually means there is an order of preference, such that it won't switch to the 2nd choice unless the 1st drops out completely or you choose to.
More strictly, when you give them the same SSID and password, the security / encryption modes also need to be the same. For example you should not have WEP on one and WPA on the other, or Open on one and encrypted on the other. This may cause an SSID to be listed twice depending on the client software because they are not interchangeable.
Where possible use WPA2-PSK (WPA2-Personal) on both, and if there is a further option, choose AES on both (rather than TKIP). Some wireless chips provide hardware acceleration for AES.
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