If you don't want to use the deprecated methods, you'll have to stop using glTexCoord2f, and use a custom attribute.
If you want to stay with immediate mode, you'll use glVertexAttrib2f.
You pair this up with your new variable 'in_coord' by quering the index after shader linking:
int program = glCreateProgram();
//create shader as normal
int texcoord_index = glGetAttribLocation(program, "in_coord");
glBegin(GL_QUADS);
glVertexAttrib2f(texcoord_index, 0, 0); ...
If you want to use VA/VBO instead, the functions you use instead is glEnableVertexAttribArray/glVertexAttribPointer
In general, the term "multi-pass rendering" refers to rendering the same object multiple times with different shaders, and accumulating the results in the framebuffer. The accumulation is generally done via blending, not with shaders. That is, the second shader doesn't take the output of the first. They each perform part of the computation, and the blend stage combines them into the final value.
Nowadays, this is primarily done for lighting in forward-rendering scenarios. You render each object once for each light, passing different lighting parameters and possibly using different shaders each time you render a light. The blend mode used to accumulate the results is additive, since light reflectance is an additive property.
Does subroutines require GLSL 4.0+? So it unavailable in GLSL version of OpenGL ES 2.0?
This is a completely different question from the entire rest of your post, but the answer is yes and no.
No, in the sense that ARB_shader_subroutine is an OpenGL extension, and it therefore could be implemented by any OpenGL implementation. Yes, in the practical sense that any hardware that actually could implement shader_subroutine could also implement the rest of GL 4.x and therefore would already be advertising 4.x functionality.
In practice, you won't find shader_subroutine supported by non-4.x OpenGL implementations.
It is unavailable in GLSL ES 2.0 because it's GLSL ES. Do not confuse desktop OpenGL with OpenGL ES. They are two different things, with different GLSL versions and different featuresets. They don't even share extensions (except for a very few recent ones).
Best Answer
There's a nice tutorial on this in the web site to go with the book OpenGL ES 2 The examples from the book are all at www.opengles-book.com.
Chapter 9, Simple_Texture2D does exactly what you want. It sets up a shader that samples a texture, initializes it, and shades the triangles using the texture.
The shader program is close to:
and you set it up thusly:
But see the actual code, from the links I gave above, to really see the example.