How To Access Two Databases in a Single connection String in C#?

You use two separate connection objects:

SqlConnection con1 = new SqlConnection(@"Data 
Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\sms.mdf;Integrated 
Security=True;Connect Timeout=30;User Instance=True");

SqlConnection con2 = new SqlConnection(@"Data 
Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\sms2.mdf;Integrated 
Security=True;Connect Timeout=30;User Instance=True");