If I am putting a semi transparent face on top of another face, the transparency lost.
The left shamrock is 1mm above the table, the right one is actually on top of the face composing the table.
Is it intended behaviour? Any idea how to make the second face actually semitransparent please?
No. It’s a limitation of your graphics card’s ability to figure out which face is in front of which when you put them in the exact same location. Don’t do that. In reality the two faces could not be in the same place at the same time. What is the actual intent here? Maybe it makes more sense to use the outline of the shape to divide the larger face into two regions and then use a different material inside as compared to utside.
Setting the Transparency Quality to Nicer in your style settings also helps with some of these problems, especially with semitransparent PNG images. But placing two faces at the same plane or very near each other will produce a flickering z-fighting effect.
If you need it for rendering (V-Ray), create a Blend material and apply it on the desired (delimited) area.