I am currently experimenting with dual-layer ASCII on the Casio CFX-9850G calculator to see if it works and it appears to do so, although some colors behave differently than others. For example, when recalling the picture, blue has priority over green and orange has priority over everything else.
ooo
+
ooo
=
ooo
ooo
+
ooo
=
ooo
ooo
+
ooo
=
ooo
And yeah as you might guess, recalling a pic doesn't use the overwrite logic, so pixels that are turned off in the pic won't turn OFF pixels in the screen when recalled. This means dual-layer ASCII is possible on this calc, although the colors have different priority on each others and only small fonts can be used on graph screen.
I'll probably try to experiment more when I am bored. I wonder if BASIC pics will behave the same way on the prizm, if available...