Can you have a SQL dual connection string

Take a look here:
How to connect with two different databases by one connection string Sql and Vb.Net - Stack Overflow[^]

If the "two different servers" case in the above answer is your case, then I'm afraid that there is no alternative than having two different SqlConnections with different connection strings.

As pointed out, if you want to communicate with two different databases from a client program, you need two connections thus two connection strings.

However, since you're moving data from one database to another, I would consider not transferring the data via a client program. When you use a client program you first need to read the data into the program, perhaps process it and then write it to the other database.

Instead, I would suggest using a linked server. A linked server is a server side object that can be used to communicate directly from a database to another. I believe that this approach would be more straightforward and also more efficient. In the best case you could perhaps use a simple INSERT INTO ... SELECT FROM structure to transfer the data.

For more information, have a look at Linked Servers (Database Engine) | Microsoft Docs[^]