Insert soulstones into vending machine [Help]

[Soesjes]

Hi, I've got a question.
I have come across many vending machines where I have to insert 2 soulstones each time. (There's only 1 on a map)
However nothing happens. I keep inserting 2 soulstones every time I come across one, thinking that maybe I have to wait and do it a lot of times for something to happen but I think I may have inserted about 30 right now and feel like nothing will happen.
Am I doing something wrong, is it just a trap to get nothing in return or am I missing something?
Kind regards Soesjes

[Vengyr]

You need to do it twice in the same playthrough, on Garden 2 (where you face Witch/Golem/Dorchi) and Circus 1 (where you face Tank/Joker). After inserting both, somewhere on Circus 1 map a door will appear (you can hear it by the wind sound). You will need 2 keys to unlock it, so plan ahead.
This allows you to access the Crimson Wilderness area, for which while camping after finishing Circus 1 area you go not to the right, but up.

[Soesjes]

Thank you, I just got an interaction with hedgehog sir Doidoichi which explained it as well. It's strange then that the option is already there even before the dialogue. Anyways I completed the game and it's freaking amazing! I love it so much