It depends upon the tape drive, and the software used. Most tape drives have the ability of writing special "markers" on to the tape in order to keep track of multiple archives. As Matir stated, tape drives hold a contiguous data stream and have no file system. However, it is possible to write multiple archives on a tape by using (if available) drive firmware that writes out special "markers". One of the "markers" is an "end-of-block", or EOB marker. Another common "marker" is an "end-of-tape", or EOT marker. The EOB marker is used to mark the end of one contiguous stream called an archive, and is written when the original output stream has been closed. The EOT marker is used to mark the logical end of the tape and is written when the tape unit is finally closed at the end of a complete backup session.
All of this long winded verbiage was an attempt to say that yes it is possible to write multiple archives on to a single tape, but that it really depends upon the tape drive (and what the firmware allows), along with the software that may be capable of using and understanding those tape markers.
There are a number of DAT tape drives and general purpose UNIX utilities that provide this support, but it has been too long since I have used them and do not remember the names, sorry.