quick example

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.Graphics;

namespace WindowsGame1
{
    public class Game1 : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;

        RenderTarget2D MyRenderTarget;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        protected override void Initialize()
        {
            base.Initialize();
            MyRenderTarget = new RenderTarget2D(GraphicsDevice, 100, 100);
        }
        protected override void LoadContent()
        {
            spriteBatch = new SpriteBatch(GraphicsDevice);
        }

        protected override void UnloadContent()
        {
        }

        protected override void Update(GameTime gameTime)
        {
            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {

            GraphicsDevice.SetRenderTarget(MyRenderTarget);

            // draw the first shader pass with your textures here, like normal using spritebatch.

            GraphicsDevice.SetRenderTarget(null);

            //Draw the second pass here, using MyRenderTarget as the source texture.

            base.Draw(gameTime);
        }
    }
}