with Sense I have two SSID (before I never used an "ac" Wifi hotspot, maybe it's normal)
I have a <mySSID> SSID (!!!) and another one <MySSID>_5GHz
Why this second one ?
Most routers now come with "dual band" as standard, meaning they operate both within the 2.4GHz and 5GHz ranges. The most commonly used is 2.4GHz, designated with just the SSID label, whereas the "faster" 5GHz, as shown in your SSID_5GHz, is usually either hidden or not enabled in most out-of-the-box settings. The Sense just broadcasts both these ranges by default you first set up the device.
There appears to be now way of turning this off, or renaming the SSID_5GHz to something else - as you've probably noticed, the app is fairly simple...
Quoting @FS_Simo's reply here, SENSE provides two WIFI networks, one running on the 2.4 Ghz band and another at 5 GHz, the 5 GHz should provide a significantly faster connection.
Why not both on the same SSID?
Because if your device can see both 2.4 and 5, then it is easier to connect to the right one.