asp.net c# gridview - individual row checkboxes for duplicates but final checkbox when satisfied

I am trying to write a meter tick and billing program in C# ASP.NET Webforms for a photocopier company. Some photocopiers have a B&W has as a Color meter. My Sql query presents 2 rows for the same photocopier so I may edit the monthly B&W and Color reads individually as they are billed at different rates. I wish to have one Invoice/Bill that shows both meter reads along with the final bill. Thus, until both meter reads have been entered it should not post. Once both the checkbox rows have been satisfied I require a third checkbox that checks them and then shows checked so they can be posted.