Excel spreadsheets combine into 1 File

I am using Excel 2016. I came across an Excel Spreadsheet file. When I open it, it opens 2 Excel Spreadsheets (file1.xlsx and file2.xlsx).
I wanted to ask how you can have 2 different spreadsheet files, combined into 1 Excel file. I cannot upload the file here. Please let me know if you have ideas on how this can be done Thanks for any help

I went to search further and realized I made mistake in filename, it was actually a : colon at the end then a number. I read in other posts here is the cause/resolution:
"You have multiple windows of the same workbook open so you can work in multiple areas. Control W will close the extra window. Go to the workbook you want to close and press Control W." So I am ok