I don't know about #2.

For #1, I have been involved with a large application that was in 2 separate
solutions in VS 2003, then combined into one big solution for VS 2005.

The two parts are referred to as the "platform" and the "application". We
could write additional applications that use the common platform, but today,
there is just the one application.

Logically, I like to see the code in two separate solutions, because it
makes the developers think harder when making calls between the app and the
platform. For example, the platform never calls the application (but the
application may call the platform).

However, at least in VS 2005, it's easier to add references and debug when
everything is in one solution. On the whole, it seems preferable to put all
the code into one solution when you have access to all the source code.

Gary