Access to previous frames

...
texture texColorBuffer : COLOR; // or SV_Target

texture	texCurr1
{
	Width 		= 	BUFFER_WIDTH;
	Height 		= 	BUFFER_HEIGHT;
	Format 		= 	RGBA8; 
};

texture	texPrev1
{
	Width 		= 	BUFFER_WIDTH;
	Height 		= 	BUFFER_HEIGHT;
	Format 		= 	RGBA8; 
};

texture	texPrev
{
	Width 		= 	BUFFER_WIDTH;
	Height 		= 	BUFFER_HEIGHT;
	Format 		= 	RGBA8; 
};

sampler samplerColor
{
	Texture 	= texColorBuffer;
	AddressU 	= CLAMP;
	AddressV 	= CLAMP;
	AddressW 	= CLAMP;

	MinFilter 	= LINEAR;
	MagFilter 	= LINEAR;
	MipFilter 	= LINEAR;

	MipLODBias 	= 0.0f;
	
	MinLOD 		= -1000.0f;
	MaxLOD 		= 1000.0f;
	
	MaxAnisotropy 	= 1;
	
	SRGBTexture 	= FALSE;
};

sampler	samplerCurr1
{
	Texture		= texCurr1;
};

sampler	samplerPrev1
{
	Texture		= texPrev1;
};

sampler	samplerPrev
{
	Texture		= texPrev;
};

float4	currprevTest(in float4 pos : SV_Position, in float2 texcoord : TEXCOORD0, out float4 curr : SV_Target0, out float4 prev : SV_Target1)
{
		curr	=	tex2D(samplerColor, texcoord.xy);
		prev	=	tex2D(samplerPrev, texcoord.xy);
}

float4 	MainPS(in float4 pos : SV_Position, in float2 texcoord : TEXCOORD0, out float4 color : SV_Target0, out float4 prev : SV_Target1)
{
		color		=	tex2D(samplerColor, texcoord.xy);

		// stuff...

		color.w		=	1;
		prev		=	color;
}

technique Effect < enabled = true; >
{

	pass currprevPass
	{
		VertexShader 	= MainVS;
		PixelShader 	= currprevTest;

		RenderTarget0	= texCurr1;
		RenderTarget1	= texPrev1;
	}
	pass finalPass
	{
		VertexShader 	= MainVS;
		PixelShader 	= MainPS;

		RenderTarget0	= texColorBuffer;
		RenderTarget1	= texPrev;
	}

}