Art,
This is exactly what I was solving on NYE.
http://srforums.prosoundweb.com/index.php/t/52412/16267/In my case the resulting pattern of the two tops (irregardless of whether it was run in stereo or mono), when configured and arrayed as described, was 90 degress horizontal.
People (meaning: the patrons and the owners) really don't care "how it looks" if it sounds like it should. Meaning: Sounds clear.
Multiple sources with multiple reflections are a bad thing.
Unless I have a very wide venue (meaning wider than deep) I use a single stack, and it is NOT against a wall.
{soapbox turned on}
When are we gonna lose this "gotta be left+right" deployment in small venues?
{soapbox turned off}
-Dennis Wiggins