For convenience, you can link your corona project files into visual studio instead of making a separate copy of them. This way you can have one file base instead of having to copy updates into or out of your windows phone project when you make changes to your code.
To do this:
* Right click on the Assets/Corona then click Add->Existing Item
* In the popup window select your corona project files
* In the bottom right corner click the down arrow on the Add button
* Click Add As Link
Now you have one code base!