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Use the following layout:

```cmake
project/
    CMakeLists.txt
    project_1.cpp
    project_2.cpp
    libA
        CMakeLists.txt
        libA_1.cpp
        libA_2.cpp
    libB
        CMakeLists.txt
        libB_1.cpp
        libB_2.cpp
```

In `project/CMakeLists.txt`, you would have:

```cmake
cmake_minimum_required(VERSION 3.16) # or newer
project(project)

# Set a global C++ version
set(CMAKE_CXX_VERSION 14)
set(CMAKE_CXX_VERSION_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS NO)

# Add libraries to the build
add_subdirectory(libA)
add_subdirectory(libB)

# Create main project executable and link to libs
add_executable(main project_1.cpp project_2.cpp)
target_link_libraries(main PRIVATE libA libB)
```

Then both `project/libA/CMakeLists.txt` and `libB` will look like:

```cmake
# Create "libX" (X=A or B) library
add_library(libX libX_1.cpp libX_2.cpp)

# Expose headers to internal build.
target_include_directories(
    libX PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>)
```

Then, when building, you can select an out-of-tree directory for _everything_. CMake will automatically create a parallel directory structure (that's part of what `add_subdirectory` does) for the sub-builds.

```console
$ cd /path/to/project
$ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -S . -B build
...
$ cmake --build build
...
$ ./build/main
...
```

As of CMake 3.19, you can use [presets](https://cmake.org/cmake/help/latest/manual/cmake-presets.7.html), which remove most needs for wrapper scripts that invoke CMake. See the documentation for more details.

Also, the build directory has never needed to exist before calling CMake; it will create it if it does not exist.