Can I overlap two framebuffer attachments outputs in a fragment shader? - vulkan

Question

Right now I am writing out to a colour buffer in the fragment shader, which is a float format.
layout (location = 0) out vec4 outColour;
I need to have a way to write the object's id to a framebuffer for picking. There are a number of ways I've thought about doing this. I can compile two versions for each shader, one a normal one, and another for the picking, which basically only needs to do the vertex position transformations and then skip everything else, lighting calculations, texturing, etc. This probably isn't ideal because this is essentially doubling the number of shaders I have to write.
An easier method I've thought is to do a conditional branch (preferably over a specialisation constant), and for picking purposes compile a picking version of the graphics pipeline with the picking boolean value set to true. This sounds better. For the ordinary passes I can write to multiple attachments. Will it be best to compile that picking pipeline with a new render pass that writes to only one framebuffer attachment, an integer one? If I swap the render pass for one that writes an integer at attachment 0 instead of the float 4 can I alias this in the fragment shader?
layout (location = 0) out vec4 outColour;
layout (location = 0) out ivec4 out_id;
void main()
{
vec4 colour;
int object_id;
if (bPicking)
out_id = ivec4(object_id, 0, 0, 0); // y, z, w not used
else
out_colour = colour;
}
I'm guessing I really need a different render pass because instead of writing to a R32G32B32A32_SFLOAT image I'm writing to a R8_UINT image for the IDs. This is really confusing what's the best way to do this?