const GLenum attachments[] = {GL_COLOR_ATTACHMENT1};
glDrawBuffers(1, attachments);

// Assuming the FBO and the 2 requried textures were correctly generated

glBindFramebuffer(GL_FRAMEBUFFER, _fbo);
glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, _fbo_tex[0], 0);
glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, _fbo_tex[1], 0);

// glCheckFramebufferStatus(GL_FRAMEBUFFER) returns GL_FRAMEBUFFER_COMPLETE

#version 300 es

uniform mat4 modelviewProjectionMatrix;
in vec4 position;

void main() {
    gl_Position = modelviewProjectionMatrix * position;
}

#version 300 es

uniform lowp vec4 colorIn;
layout(location = 0) out lowp vec4 colorOut;

void main() {
    colorOut = colorIn;
}

// Assuming the correct program and vertex array is bound

glBindFramebuffer(GL_DRAW_FRAMEBUFFER, _fbo);
{
    const GLenum attachments[] = {GL_COLOR_ATTACHMENT0};
    glDrawBuffers(1, attachments);    // OK
    glClear(GL_COLOR_BUFFER_BIT);

    // Draw red quad to GL_COLOR_ATTACHMENT0
}
{
    const GLenum attachments[] = {GL_COLOR_ATTACHMENT1};
    glDrawBuffers(1, attachments);    // GL_INVALID_OPERATION Error
    glClear(GL_COLOR_BUFFER_BIT);

    // Draw green quad to GL_COLOR_ATTACHMENT1
}

const GLenum attachments[] = {GL_NONE, GL_COLOR_ATTACHMENT1};
glDrawBuffers(2, attachments);    // OK
glClear(GL_COLOR_BUFFER_BIT);