Changing Staff organization

[Alberto Simões]

Hello

I have a music that starts with two staffs, and at some points splits to
three (for four voices).
Is there any way to force this change?

Or is the solution to make two separate melodies?

Thanks for any hint and link

Cheers
Alberto
-- 
Alberto Simões - Departamento de Informática - Universidade do Minho
                 Campus de Gualtar - 4710-057 Braga - Portugal