How to structure cmake project with 2 main src files - c++

Question

I would like to understand how to structure my cpp project correctly. I am using the build generator CMAKE. The build system I am using is Ninja. I have 2 main functions in my project. Each main should be compiled into a different executable.
When and why should I use multiple cmake files?
How can I better structure my project?
|-- CMakeLists.txt
|-- README.md
|-- env.csh
|-- include
| |-- Pen.h
| |-- Cup.h
| |-- Clip.h
| |-- Fun.h
| |-- Ins.h
| |-- Ne.h
| `-- Pa.h
|-- libs
|-- src
| |-- Pen.cpp
| |-- Cup.cpp
| |-- Clip.cpp
| |-- Fun.cpp
| |-- Ins.cpp
| |-- Ne.cpp
| |-- Pa.cpp
| |-- main0.cpp
| `-- main1.cpp
`-- tests
`-- test.cpp

Answer 1

You need one add_executable() line for each executable in your project. Try this CMakeLists.txt file (written mostly from memory):
cmake_minimum_required(VERSION 2.8)
project(myproject LANGUAGES CXX)
enable_testing()
include_directories(${CMAKE_CURRENT_SOURCE_DIR}/include)
set(SOURCES
src/Pen.cpp
src/Cup.cpp
src/Clip.cpp
src/Fun.cpp
src/Ins.cpp
src/Ne.cpp
src/Pa.cpp
)
add_executable(main0 src/main0.cpp ${SOURCES})
add_executable(main1 src/main1.cpp ${SOURCES})
add_executable(unittests tests/test.cpp ${SOURCES})
add_test(tests unittests)