Shifting Texture Coordinates via CG Shader

Hi All,

I am in the process of learning Shader scripting using CG following the nVidia tutorials. I am trying to replicate the double-vision texturing as can be seen here in section 3.5.5:

http://http.developer.nvidia.com/CgTutorial/cg_tutorial_chapter03.html

I am getting an error in the Unity Console stating:

Program 'frag', ambiguous overloaded function reference "tex2D(sampler2D, float4)" at line 27

My code for the shader is:

 Shader "Custom/RGBSplat" {
     Properties {
     _MainTex ("Texture", 2D) = "white" { }
     _Seperation ("Seperation", float) = 0.0
 }
 SubShader {
     Pass {
 
 CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 
 #include "UnityCG.cginc"
 
 sampler2D _MainTex;
 float _Seperation;
 
 void vert (appdata_full v, out float4 oPosition : POSITION, out float4 leftTexCoord : TEXCOORD0)
 {
     
     oPosition = mul (UNITY_MATRIX_MVP, v.vertex);
     leftTexCoord = v.texcoord;
 }
 
 void frag (float4 leftTexCoord : TEXCOORD0, float4 rightTexCoord : TEXCOORD1, out float4 oColor : COLOR)
 {
     float4 leftColor = tex2D(_MainTex, leftTexCoord);
     //float4 rightColor = tex2D(_MainTex, rightTexCoord);
     //oColor = lerp(leftColor, rightColor, 0.5);
     oColor = leftColor;
 }
 
 ENDCG
 
     }
 }
 Fallback "Diffuse"
 }
 

I cannot seem to figure out what the problem is, if any scripting guru out there has an idea.

Also it is worth noting that I see a lot of people have said to animate/shift texture UV coordinates from within a MonoBehavior script. But I would like to do this in a shader. And wouldnt it be faster running on the GPU than the CPU for manipulating texture UVs?

Thanks!