Figure out which cells are merged in a streaming fashion

Hi,

I am trying to parse large (many sheets with hundreds of thousands of rows and tens of columns) spreadsheet files, xlsx and xlsb, in a streaming fashion. The spreadsheet is a standard table with a two-row header, where the upper row are merged cells representing groups of columns, and the lower row contains names of columns within a group. I need to identify the columns I need to parse before reading the data, and for that I need to know which group a given column belongs to.

However, I cannot find information on how to recognize merged cells in a streaming fashion. Is it even possible? As far as I know, in case of xlsx files, information on which cells are in a merged region is stored in the <mergeCells> element of the sheet xml, and in my example files this element is at the end of the file.

Thank you,