You can create multiple import templates (for sheet1, sheet 2, etc). Upload an excel file and run the excel importer microflow with the templates for sheet 1, 2 etc. Change the assocation in a mf to the template before importing the file.
Edit 1: You must leave the association as it is now, but create a microflow based on IVK_ImportTemplateDocument
After StartImportByTemplate retrieve a sheet2 template and callStartImportByTemplate again with that template.
Repeat when needed with the next template.
The association TemplateDocument_Template is used to choose a template in the user interface, but you can select a template in a microflow in any possible way.