Can I create an application that can work on both windows platforms (32 / 64 bit ) at the same time ? if no , how can i create 64bit version and 32bit version ?
Yes you can - just set the compile option in Vizual Studio to AnyCPU (Build>Configuration Manager>Active Solution Platform). To Target 32bit or 64 bit set the platform to x86 or x64 respectively
Have a look at this link explaining the differences
But AnyCPU is not necessarily the best option - so have a look at this article: AnyCPU is it evil and make up your own mind what setting you would like
If you are using Visual Studio Express then have a look at this question and answers explaining how to do this