I've seen this idea a few places now:

"Each implementation of a DebuggerVisualizer can be associated with only one type."
"Debugger visualizers for different classes must reside in a separate project."

And I'm really not sure what to make of it. What we call a "debugger visualizer" is a loose constellation of classes working together, communicating via streams and method calls, brought together by a DebuggerVisualizer attribute. It is true that per DebuggerVisualizer attribute, only one target type can be specified. But the same set of classes can be specified in any number of DebuggerVisualizer attributes, and any one assembly can have multiple assembly-level DebuggerVisualizer attributes.

So, to be completely clear: one assembly/project/set of classes can support any number of target types.

[see my visualizers category for discussion/documentation and links to other visualizers]