Instruction

Let's take for example the situation where you need to combine four materials with square textures into one

Create a new material and add a merged texture to it

Create UV Editing tab, turn off UV Sync Selection and change Pivot to 2d cursor (make sure the cursor is in the lower left corner, if it is not, press "Shift+C" to return it to its original position)

Select the merged texture

Select old material and press "Select"

in the UV editor select all vertices with button "A", then press "S" to scale, and write on the numpad "0.5" to halve it

To move to the desired position, press "G" then "X" or "Y" (or middle mouse button) to move along the desired path, and also write 0.5 on the numpad for exact location. Repeat for all materials.

When everything is ready you can attach the materials to the new one. (In my case, I could just delete the old materials)