Send Multiple Attachments With Send DBMail
The semi-colon is a statement terminator, and you haven't correctly escaped your attachments. In other words, you've done this:

EXEC msdb.dbo.sp_send_dbmail
  @profile_name = 'Mod',
  @from_address = 'modis@modisglobal.com',
  @recipients= 'rsmith@gmail.com',
  @subject= 'Test Email', 
  @body = @body1,
  @file_attachments = @filename1;

@filename2; -- Syntax Error


According to the MSDN article on sp_send_dbmail:


  [ @file_attachments= ] 'file_attachments'
  
  Is a semicolon-delimited
  list of file names to attach to the e-mail message. Files in the list
  must be specified as absolute paths. The attachments list is of type
  nvarchar(max). By default, Database Mail limits file attachments to 1
  MB per file.


You need to do this:

Declare @filenames varchar(max)
Set @filenames = 'C:\Testfiles\Test1.csv;C:\Testfiles\Test2.csv'

EXEC msdb.dbo.sp_send_dbmail
  @profile_name = 'Mod',
  @from_address = 'modis@modisglobal.com',
  @recipients= 'rsmith@gmail.com',
  @subject= 'Test Email', 
  @body = @body1,
  @file_attachments = @filenames;


If you want to programmatically add attachments, you would do something like this (notice how the second and third files begin with a semi-colon):

DECLARE @filenames varchar(max)
DECLARE @file1 VARCHAR(MAX) = 'C:\Testfiles\Test1.csv'
SELECT @filenames = @file1

-- Optional new attachments
DECLARE @file2 VARCHAR(MAX) = ';C:\Testfiles\Test2.csv'
DECLARE @file3 VARCHAR(MAX) = ';C:\Testfiles\Test3.csv'

-- Create list from optional files
SELECT @filenames = @file1 + @file2 + @file3

-- Send the email
EXEC msdb.dbo.sp_send_dbmail
  @profile_name = 'Mod',
  @from_address = 'modis@modisglobal.com',
  @recipients= 'rsmith@gmail.com',
  @subject= 'Test Email', 
  @body = @body1,
  @file_attachments = @filenames;