SQL Connection String

I have created an application using Visual Studio with a SQL database. I tried to run the program simultaneously with the help of my friend. The problem I encountered is that, the last application who tries to connect to the database receives this error message: “Not allowed to change the ‘ConnectionString’ property. The connection’s current state is open.”

It seems like whenever the connection’s state is open it can never be opened again by another application unless the connection’s state is close.

Can you help me solve this issue? I’m a newbie and your help is highly appreciated. Thank you in advance.