The Designer.cs file is just a partial class and you could split this
into multiple files using the same principle... but your designer is
only ever going to be putting it's changes into one of these. So the
answer, in short, is no you can't do what you want to.

The only solution available to you is to merge the changes when designer
number 2 checks his changes into the VCS. In a few situations these
changes might be incompatible (if both designers were modifying
properties/events of the same components) but generally speaking a
descent file compare tool should let you merge changes without any
problem (although not without a considerable amount of effort in the
case of the Designer.cs file). If the compare tool you're using at the
moment sux I could recommend CompareIt! (which I've been using for years
- very slick).

I would have to say that what you're trying to do is basically not a
good idea though. If you have multiple developers working on different
aspects of the same interface, you should try to split the interface
into components and have each developer build only the component(s) that
he/she is responsible for. If that's still not enough flexibility for
you then you could go for a plugin based approach/framework - which
gives you the advantage of a formal contract that you can define to
specify the interaction between one developer's components/plugins and
another's.

Best Regards,

James Crosswell
Microforge.net LLC
http://www.microforge.net