I know I can use 8BitBG as background like this:
BGTopText will be drawn on top of BGTop8Bit.

Now, I'd like to use two 8BitBG like this:

However, BG1Top8Bit does not be drawn on top of BGTop8Bit.
Whichever BG8Bit (BG1Top8Bit or BGTop8Bit) I use to draw something, it is drawn on top.
I can't use one as a background, another as a graphic.

And I use BGBot8Bit:Clear() to clear BGTop8Bit, but BG1Top8Bit is also cleared.
I doubt I can't use 8BitBG more than one at a time.

Do anyone know about this? Please let me know.
Sorry for newbie question.

EDIT
This is the sample code.
http://atbb.jp/takeru/viewtopic.php?t=36&start=43