Help with tar cmd for directories

Hi, I'm working on HP-UX B.11.23 64bit. I tried to tar couple of directories but failed to do so.
$ tar -cvf tar_file_name -C /dir1 /dir2

the -C is for directories as mentioned in the man pages. But still unable to create a tar file having directories and sub-directories.
Requesting help in this regard.

$ tar -cvf tar_file_name -C /dir1 . -C /dir2 .

Note the <space> between /<dir> and .