using System;
using System.IO;
string filetomerge=@"C:\temp\data.bin";
string targetpath=@"D:\store";
string strFileName = filetomerge.Substring(filetomerge.LastIndexOf(Path.DirectorySeparatorChar) + 1);
FileStream fsr1 = new FileStream(targetpath+ Path.DirectorySeparatorChar + strFileName + "1", FileMode.Open, FileAccess.Read);
FileStream fsr2 = new FileStream(targetpath+ Path.DirectorySeparatorChar + strFileName + "2", FileMode.Open, FileAccess.Read);
byte[] btArr = new byte[fsr1.Length+fsr2.Length];
fsr1.Read(btArr, 0, Convert.ToInt32(fsr1.Length));
fsr2.Read(btArr, Convert.ToInt32(fsr1.Length), Convert.ToInt32(fsr2.Length));
fsr1.Close();
fsr2.Close();
FileStream fsw = new FileStream(%%2 + Path.DirectorySeparatorChar+ strFileName, FileMode.Create, FileAccess.Write);
fsw.Write(btArr, 0, btArr.Length);
fsw.Flush();
fsw.Close();