Windows doesn't allow you to sleep specific monitors only :(
The other thread where you create a Monitor Profile with monitors 1 and 2 disabled, and just toggle between that is probably your best option. If you need help configuring that, please let me know.
Alternatively, you could use the Monitor Fading feature to make the two monitors blank, but this doesn't actually sleep them, it just overlays a black window.

Nothe1
2 discussion posts
Thanks for the reply man👍 I have been using the fading feature for a bit now as it seemed like the best option. Unforunatly it doesn't turn off the backlight like sleep does. I guess if they were OLED monitors that would solve the problem lol $$.
Haha yeah, they're not cheap!