GLuint hProgram;  // already created
GLuint hVertexShader1, hVertexShader2; // created and compiled
GLuint hFragmentShader1, hFragmentShader2; // created and compiled

glAttachShader( hProgram, hVertexShader1 );
glAttachShader( hProgram, hFragmentShader1 );
glAttachShader( hProgram, hVertexShader2 );
glAttachShader( hProgram, hFragmentShader2 );

// then link the program to get ready for use
glLinkProgram( hProgram );

...

// activate the program when rendering
glUseProgram( hProgram );
... // draw your stuff
glUseProgram( 0 ); // disable shaders and reenable fixed pipeline