Kontakt plays every loaded instrument at the same time.

Try this alternative:

Use Host Automation to host the S and M buttons.

When you want to hear the first instrument only, silence the second instrument with the automation, and vice versa.

Or try the alternative of the first alternative:

Use host automation for the volume, so that when you wanna hear the first instrument, use the volume host automation to silence the other instrument.

Let me know if it helped.

I will explain it to you tomorrow how can you do it with couple of screenshots, i am going on a school vacation to Turkey.

When I come to the hotel, I will screenshot the solution.

Oh, nice!

Expect the screenshot explanation by either tomorrow or 2 days!

That's how it's intended to work. Whatever instruments are loaded in the rack will all play at the same time.

You can solo an instrument just like in most DAW's and that will mute the rest. Alternately you can mute specific instruments and play multiple other instruments.

Not sure this is the answer you wanted to hear, but this is how it works currently.

I tested it using the default settings that Kontakt gave me, I didn't assign anything or change any of the stock settings. Perhaps in previous versions it would automatically assign each instrument in a rack to a new MIDI channel? Or there's a menu setting that you can enable to have that done automatically?

I found more information while searching for a fix for a different problem. There is a menu setting that you can change so it will automatically assign each rack instrument to a different MIDI channel automatically. By default it's set to Assign to omni. But you can change to to Assign first free, if you'd rather not have instruments all on the same channel.