(Summary of discussion)
You have 28.3 GB left in the partition that holds your current directory, and the second part of the archive is 27.8 GB. This means that you should be able to append the second part of the archive onto the first part of the archive.
Assuming the two parts are called
cat part2 >>part1
part1 would be the complete archive file (and
part2 could be deleted).
The only issue is that there usually is a reserved buffer of space on a Unix filesystem that ordinary users can't fill (it's usually a few percent of the total partition size). Since the size restrictions here are so tight, I'm not entirely sure you would be within these size limits.
The remedy for that is either to run as the root user (who's not restricted in filling up the reserved space), or to simply clear out some large unneeded files.
To run as root, either bring up a root shell with
sudo -s and run the
cat as above, or do
sudo tee -a part1 <part2 >/dev/null