Auto Increment Key Pair with Start Digit at 200000 SQL SERVER 2008
Here is one way, note that this effectively limits concurrency, so don't expect stellar performance at scale.

INSERT dbo.TableName(col1, col2)
  SELECT @ClientName, COALESCE(
    (SELECT MAX(col2) + 1 
     FROM dbo.TableName WITH (UPDLOCK, HOLDLOCK)
     WHERE col1 = @ClientName), 200000);


Of course if anyone ever deletes any rows from this table, you'll be missing values. And if an insert gets rolled back, that rolled back number will be reused, so be careful if you reveal that assigned value anywhere prior to the rollback.