Opengl shader GLSL blend two texture by coord

Question

How can I blend two texture so the texture are side by side, With 50 % of texture A on the left, and 50% of texture B on the right.

for example

----------------
|       |      |
| 50% A | 50% B|
|       |      |
----------------

I want the region to be changeable, so might be 25% A 75% B.

Now I can blend two textures fully on the Rect, with each being translucent.

this is my frag shader

uniform sampler2D tex;
uniform sampler2D texx;
uniform float alpha;
in vec2 textureCoord;
out vec4 finalColor;

void main() {
    vec4 first_sample = texture(tex, textureCoord);
    vec4 second_sample = texture(texx, textureCoord);

    finalColor = first_sample * alpha + second_sample * (1-alpha);
}

Do I need to create an alpha mask texture in my C++ program? Thank you.

Answer 1

Alpha mask (1D texture would be enough) is one of possible solutions, but in this particular case it isn't necessary

uniform sampler2D tex0;
uniform sampler2D tex1;
uniform float t;    // e.g. 0.4
in vec2 texcoord;
out vec4 outColor;

void main(void)
{
    vec4 t0 = texture2D(tex0, texcoord);
    vec4 t1 = texture2D(tex1, texcoord);
    outColor = mix(t0, t1, step(t, texcoord.x));
}

By the way, it isn't called 'blending'. Rather 'threshold' or 'step'.