Where I put a second image in my Vu (to enable multiboot) ?

Look at the screen, you have 3 slots/directories with no image in them. And you have a recovery image (ATV). A recovery image is meant to be emergency use.

I would flash ATV in slot 1 for ATV daily usage. Then VTI in slot 2 or 3, it doesn't matter which. Once you have booted into VTI, don't forget to install the multiboot selector plugin

For flashing:

You must use "flash online" to flash the image, it will then ask you which slot to use.

For booting:

There is a multiboot option in the standby/restart menu which allow you to choose which one to boot into. VTI does not support multiboot, so you must install the multiboot selector plugin after you boot into VTI.

Slots/directories:

The slots/directories were created when you enabled the multiboot function. Your screenshots show them already, you don't need to do anything.