So I guess there are two interpretations of “displaying several notes at the same time.” Fortunately, I have both covered.
First, to display several notes separately (in their own Marked windows), just check the $MyBoolean box on multiple notes. They’ll each open in separate windows in Marked – though on my machine they overlap, so I have to move one window out of the way to see them both.
Yep that one is a bit trickier. There’s a bit of incompatibility between Markdown formatting and $HTMLMarkupText (the thing that processes things like ^include^). So my solution was to enable $HTMLMarkupText for my notes – which means I lose Markdown preview inside of Tinderbox, but that’s okay because I’m using Marked to preview it anyway 
The changes are complex enough that think it warrants its own thread: