What I understand is you want same connection string with different Metadata in it. So you can use a connectionstring as given below and replace "" part. I have used your given connectionString in same sequence.
connectionString="<METADATA>provider=System.Data.SqlClient;provider connection string="Data Source=SomeServer;Initial Catalog=SomeCatalog;Persist Security Info=True;User ID=Entity;Password=SomePassword;MultipleActiveResultSets=True""
For first connectionString replace <METADATA>
with "metadata=res://*/ModEntity.csdl|res://*/ModEntity.ssdl|res://*/ModEntity.msl;"
For second connectionString replace <METADATA>
with "metadata=res://*/Entity.csdl|res://*/Entity.ssdl|res://*/Entity.msl;"
For third connectionString replace <METADATA>
with "metadata=res://*/Entity.csdl|res://*/Entity.ssdl|res://*/Entity.msl|res://*/ModEntity.csdl|res://*/ModEntity.ssdl|res://*/ModEntity.msl;"
Happy coding!