xna 4.0 sprite + MRT + pixelshader - xna

Question

I'm trying to combine 2 rendertargets, color and normal, for diffuse lightning and to render the result on screen. The idea is to use a sprite with an effect containing only a pixelshader to combine the rendertargets from textures.
XNA code:
GraphicsDevice.SetRenderTarget(null);
GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.Black, 1.0f, 0);
effect.CurrentTechnique = effect.Techniques["show_buffer"];
effect.Parameters["normalTex"].SetValue(normalRendertarget);
effect.Parameters["colorTex"].SetValue(colorRendertarget);
effect.Parameters["AmbientIntensity"].SetValue(ambientIntesity);
effect.Parameters["LightDirection"].SetValue(lightDirection);
effect.Parameters["DiffuseIntensity"].SetValue(diffuseIntensity);
spriteBatch.Begin(0, BlendState.Opaque, null, null, null,effect);
spriteBatch.Draw(normalRT, Vector2.Zero, Color.White);
spriteBatch.End();
For some reason the rendertarget used in spriteBatch.Draw() influences the result.
Pixel Shader:
void Tex_PixelShader(float2 texCoord : TEXCOORD0, out float4 color : COLOR0)
{
float4 normal = tex2D(normalTexSampler, texCoord);
//tranform normal back into [-1,1] range
normal.rgb = (normal.rgb*2)-1;
float4 baseColor = tex2D(colorTexSampler, texCoord);
float3 lightDirectionNorm = normalize(LightDirection);
float diffuse = saturate(dot(-lightDirectionNorm,normal.rgb));
//only works with normalRT in spriteBatch.Draw()
//colorRT in spriteBatch.Draw() gives colorRT but darker as result
color = float4 (baseColor.rgb * (AmbientIntensity + diffuse*DiffuseIntensity), 1.0f);
//only works with colorRT in spriteBatch.Draw()
//normalRT in spriteBatch.Draw() gives normalRT as result
//color = tex2D(colorTexSampler, texCoord);
//only works with NormalRT
//colorRT in spriteBatch.Draw() gives colorRT as result
//color=tex2D(normalTexSampler, texCoord);
// works with any rendertarget in spriteBatch.Draw()
//color = float4(0.0f,1.0f,0.0f,1.0f);
}
The alpha value in both rendertargets is always 1. Adding a vertex shader to the effect results in black. Drawing one rendertarget without any effect with spriteBatch.Draw() shows that the content of each rendertarget is fine. I can't make sense of this. Any ideas?

Answer 1

Setting the textures with GraphicsDevice.Textures[1]=tex; instead of effect.Parameters["tex"]=tex; worked. Thanks Andrew.
Changed the xna code to:
GraphicsDevice.SetRenderTarget(null);
GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.Black, 1.0f, 0);
effect.CurrentTechnique = effect.Techniques["show_buffer"];
GraphicsDevice.Textures[1] = normalRT; //changed
GraphicsDevice.Textures[2] = colorRT; //changed
effect.Parameters["AmbientIntensity"].SetValue(ambientIntesity);
effect.Parameters["LightDirection"].SetValue(lightDirection);
effect.Parameters["DiffuseIntensity"].SetValue(diffuseIntensity);
spriteBatch.Begin(0, BlendState.Opaque, null, null, null,effect);
spriteBatch.Draw((Texture2D)colorRT, Vector2.Zero, Color.White);
spriteBatch.End();
And the shader code to:
sampler normalSampler: register(s1); //added
sampler colorSampler: register(s2); //added
void Tex_PixelShader(float2 texCoord : TEXCOORD0, out float4 color : COLOR0)
{
float4 normal = tex2D(normalSampler, texCoord); //changed
normal.rgb = (normal.rgb*2)-1;
float4 baseColor = tex2D(colorSampler, texCoord); //changed
float3 lightDirectionNorm = normalize(LightDirection);
float diffuse = saturate(dot(-lightDirectionNorm,normal.rgb));
color = float4 (baseColor.rgb * (AmbientIntensity + diffuse*DiffuseIntensity), 1.0f);
}