Yes. If you go to your cpanel control panel and click on ftp manager then on ftp accounts, it will give you the ftp host and the host directory.
It is usually something like this:
ftp host:
ftp@mainsite.com<that isn't a link, don't click it.

host directory: public_html/secondsite/
Then sign in with your main user name and password that you set up when you created the site.
Little tip: don't forget the second backslash in your host directory name - that one drove me nuts for hours.
