How to extract tar archive from stdin?
Use - as the input file:
cat largefile.tgz.aa largefile.tgz.ab | tar zxf -
Make sure you cat them in the same order they were split.
If you're using zsh you can use the multios feature and avoid invoking cat:
< largefile.tgz.aa < largefile.tgz.ab tar zxf -
Or if they are in alphabetical order:
<largefile.tgz.* | tar zxf -