Vertical Hotbar? Or 2 rows / 6 six buttons?
 

Yo!

What do i have to change to make one of the 3/4-size Hotbars vertical?

or have it splitin 2 rows of 6 like
1 2
3 4
5 6
7 8
9 10
11 12

so I can have 2 of them on the side of the screen?

You should be able to just open a second hotbar and resize them to make them vertical. To make them more minimalistic you can open the Window settings and turn off the frame, drop opacity of the background frame (lets you put them side by side.

**EDIT**

Hmm.. tried it myself but the frames overlap when the icons are ligned up side by side making the button next to it useless since the other window is on top. (picture shown with frames on) Any way you can shrink the blank space on the right side of the window?

Peronally I have 2 hotkey rows at the bottom horrizontally and then 1 bank on the side thats laid out 3X4 all you have to do is resize the box..