The following method works fine when referencing a sampler in HLSL

    float3 P = lightScreenPos;
sampler ShadowSampler = DPFrontShadowSampler;
float depth;

if (alpha >= 0.5f)
{
    // Reference the correct sampler
    ShadowSampler = DPFrontShadowSampler;

    // Front hemisphere 'P0'
    P.z = P.z + 1.0;
    P.x = P.x / P.z;
    P.y = P.y / P.z;
    P.z = lightLength / LightAttenuation.z;

    // Rescale viewport to be [0, 1] (texture coordinate space)
    P.x = 0.5f * P.x + 0.5f;
    P.y = -0.5f * P.y + 0.5f;

    depth = tex2D(ShadowSampler, P.xy).x;
    depth = 1.0 - depth;
}
else
{
    // Reference the correct sampler
    ShadowSampler = DPBackShadowSampler;

    // Back hemisphere 'P1'
    P.z = 1.0 - P.z;
    P.x = P.x / P.z;
    P.y = P.y / P.z;
    P.z = lightLength / LightAttenuation.z;

    // Rescale viewport to be [0, 1] (texture coordinate space)
    P.x = 0.5f * P.x + 0.5f;
    P.y = -0.5f * P.y + 0.5f;

    depth = tex2D(ShadowSampler, P.xy).x;
    depth = 1.0 - depth;
}

// [Standard Depth Calculation]
float mydepth = P.z;
shadow = depth + Bias.x < mydepth ? 0.0f : 1.0f;

If I try and do anything with the sampler reference outside the if statement then I get the following error:

Sampler parameter must come from a literal expression

This code demonstrates that

    float3 P = lightScreenPos;
sampler ShadowSampler = DPFrontShadowSampler;

if (alpha >= 0.5f)
{
    // Reference the correct sampler
    ShadowSampler = DPFrontShadowSampler;

    // Front hemisphere 'P0'
    P.z = P.z + 1.0;
    P.x = P.x / P.z;
    P.y = P.y / P.z;
    P.z = lightLength / LightAttenuation.z;
}
else
{
    // Reference the correct sampler
    ShadowSampler = DPBackShadowSampler;

    // Back hemisphere 'P1'
    P.z = 1.0 - P.z;
    P.x = P.x / P.z;
    P.y = P.y / P.z;
    P.z = lightLength / LightAttenuation.z;
}

// Rescale viewport to be [0, 1] (texture coordinate space)
P.x = 0.5f * P.x + 0.5f;
P.y = -0.5f * P.y + 0.5f;

// [Standard Depth Calculation]
float depth = tex2D(ShadowSampler, P.xy).x;
depth = 1.0 - depth;
float mydepth = P.z;
shadow = depth + Bias.x < mydepth ? 0.0f : 1.0f;

How can I reference the sampler in this manner without triggering the error?