Selecting 5GHz channels from the drop down list when using 40MHz bandwidth

As I understand it, if I use a channel width of 40MHz in the 5GHz band, this uses two of the 20MHz channels - so (ignoring the DFS channels) that will be either channels 36 & 40, or channels 44 & 48.

However, if I want to use 40MHz and also manually set the channel, which channel do I pick from the 'Channel 5 GHz' drop-down list? My choices are 36, 40, 44 or 48. If I choose 36 for one AP and 40 for as neighbouring AP, are they, in effect, using the same frequencies?

The reason I ask is that I need to cover a large area with several APs and I want to make sure that neighbouring APs are using different channels. I suppose I could use the dynamic channel feature, but I want to know how I can set things up manually, just in case I need to.