Question: I have a project PROJECT\_A which contains mdf file database.
In the solution of this project I added one new project - PROJECT\_B.

My question is:Can I use .mdf file from PROJECT\_A in PROJECT\_B?

Answer: 
You can add .mdf file as a [Link Reference](http://support.microsoft.com/kb/306234) to project B

Right click project B and then properties > Add Reference > Existing Item > Select the .mdf file and in the add button select `Add As Link` option.
Yes, you can use single .mdf file in multiple projects. However it is not recommended since it may cause 'access denied' error. It is also a bad practice. You should be using per .mdf file per project/application.