Is csv with multi tabs/sheet possible?
CSV, as a file format, assumes one "table" of data; in Excel terms that's one sheet of a workbook. While it's just plain text, and you can interpret it any way you want, the "standard" CSV format does not support what your supervisor is thinking.
You can fudge what you want a couple of ways:
Use a different file for each sheet, with related but distinct names, like "Book1_Sheet1", "Book1_Sheet2" etc. You can then find groups of related files by the text before the first underscore. This is the easiest to implement, but requires users to schlep around multiple files per logical "workbook", and if one gets lost in the shuffle you've lost that data.
Do the above, and also "zip" the files into a single archive you can move around. You keep the pure CSV advantage of the above option, plus the convenience of having one file to move instead of several, but the downside of having to zip/unzip the archive to get to the actual files. To ease the pain, if you're in .NET 4.5 you have access to a built-in ZipFile implementation, and if you are not you can use the open-source DotNetZip or SharpZipLib, any of which will allow you to programmatically create and consume standard Windows ZIP files. You can also use the nearly universal .tar.gz (aka .tgz) combination, but your users will need either your program or a third-party compression tool like 7Zip or WinRAR to create the archive from a set of exported CSVs.
Implement a quasi-CSV format where a blank line (containing only a newline) acts as a "tab separator", and your parser would expect a new line of column headers followed by data rows in the new configuration. This variant of standard CSV may not readable by other consumers of CSVs as it doesn't adhere to the expected file format, and as such I would recommend you don't use the ".csv" extension as it will confuse and frustrate users expecting to be able to open it in other applications like spreadsheets.
If I try to save data in xls/xlsx, then I get multiple sheets in a workbook.
Your answer is in your question, don't use text/csv (which most certainly can not do multiple sheets, it can't even do one sheet; there's no such thing as a sheet in text/csv though there is in how some applications like Excel or Calc choose to import it into a format that does have sheets) but save it as xls, xlsx, ods or another format that does have sheets.
Both XLSX and ODS are much more complicated than text/csv, but are each probably the most straightforward of their respective sets of formats.
I've been using this library for a while now,
https://github.com/SheetJS/js-xlsx
in my projects to import data and structure from formats like: xls(x), csv and xml but you can for sure save in that formats as well (all from client)!
Hope that can help you,, take a look on online demo,
http://oss.sheetjs.com/js-xlsx/
peek in source code or file an issue on GH? but I think you will have to do most coding on youre own