How can I merge tar files?

[phil at uxh.cso.uiuc.edu]

I would like to know if there is a program around that can merge two or more
tar files.

I tried "cat" but it seems mere concatenation of tar files is not good
enough for "tar".

I'm trying to avoid the extra /tmp space that would be taken up by
rebuilding the trees from the tar files I have.

-phil