with combining characters one can use 2 unicode characters at single location, creating a composition of two graphics. I'm thinking about simple ascii art, I would need to first use, as wikipedia says "SQUARE WITH HORIZONTAL FILL" ▤, and then on top of that, the letter "A". How to accomplish this?
What is possible with the method you describe is a kind of abuse of diacritical marks. Fell free to play around a bit with it using following demo ( http://demo.danielmclaren.com/2015/diacriticism/ ).
Below some screenshots of the Unicode code I have got from the demo above:
Claudio.txt content copied to a terminal
Claudio.txt displayed in gedit
Claudio.txt displayed in SciTE
Sorry, but I don't know at the moment why exactly the representation of same underlying Unicode text is looking different in SciTE.
Here the HexCode of the Claudio.txt file:
Claudio.txt displayed as hex code in GHex
and here the Unicode copied/pasted to the stackoverflow answer editor box:
C̱̙̳̲͇͉̄̄̚l̝̩̩̝̺̠͞ā̝͉̩̟͆u̻͈̘̞̪̅̿d̝̄̅͆̚i͇̙͈̎̚o̘̟̎̄̄̎
As you have to be able to show ASCII art in a terminal I assume that there is no way to overlay two arbitrary Unicode characters for this purpose.