Thanks @NolanHerring. That was how I understood it since it is from the same device. However, that is not what I observed. In some of our branches, some devices are able to see SSID1, but not SSID2. I even opened the ticket about it and it didn't go anywhere. So I played around with it, and here is what I found.
1) I changed the channels from Auto to some static channels, sometimes it works. But it stopped working again after one or two weeks.
2) If I disable SSID1, and keep SSID2 enabled, the devices are still not able to see SSID2.
3) If I swap the configuration of SSID1 and SSID2, the device is able to connect SSID1 regardless SSID2 is enabled or disabled.
This tells me SSIDs in MX are not equal. It seems SSID1 has a stronger signal than SSID2. I haven't tested with other SSID#, but this is what I observed so far.
Does this make sense to you?