Convert .xls/.xlsx spreadsheets to multiple .csv's based on a list

I need to convert all sheets of a single .xls/.xlsx file to a .csv. This will be done on all .xls files in all directories and sub-directories (recursively).

Step 1: Get the sheetnames of all .xls into a .csv using:

for file in $(find . -name '*.xls' -o -name '*.xlsx');do in2csv -n "$file" > ${file%.xls}-sheetnames-list.csv; done

filename-sheetnames-list.csv can act as a list:

sheetname1
sheetname2
sheetname3

Step 2 : The code for converting a specific sheet into a .csv using in2csv is:

in2csv --sheet "SHEETNAME" filename.xls > filename-SHEETNAME.csv

How can I get every sheetname in a .xls/x and write every sheet separately for all directories containing a .xls/x ?

in2csv --write-sheets "-" filename.xls > filename-sheet1.csv filename-sheet2.csv .... gives output only on sheet1.csv, not sure how to get all sheets from this.