Write to Spreadsheet (which is the function you are using) is meant to write an entire 2D array of data to a binary stream file (read the Help for this function). As such, you need to have your (single) 2D array formed, then write it "all at once". In my (not-so-humble) opinion, calling this function "Write to Spreadsheet" is something of a misnomer, as many people (myself, included) think "Excel" when we hear "Spreadsheet" (sorry, Lotus). Note that (again, from the Help file) the created file is not Excel-compatible.
Now, if you wanted to write an Excel spreadsheet, particularly if you had the Report Generation Toolkit, then it becomes a piece of cake to add rows, to add columns, to do both, and the resulting file is readable by Excel and by LabVIEW (using the Report Generation Toolkit functions, of course).
BS