If you follow a user interface template that allows you to separate the model, view and controller (for example, MVVM or MVC), you will find that it is very simple to have two separate forms that can be developed over time as needed, with minimal changes to the rest of your code.
Trying to do the work in one form for two different presentations tends to become complicated over time (compared to the alternative, just create two forms that use the same model and controller) as user requirements develop.
UPDATE
Based on your comment ... you can also do MVC using WinForms
http://www.codeproject.com/Articles/383153/The-Model-View-Controller-MVC-Pattern-with-Csharp
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