logmerge is a free, open-source tool and powerful tool for merging two or more logfiles, which will make the multilined entries appear in the correct chronological order. This will consequently avoid any breaks of entries.
The logmerge tool also provides additional arguments that control an adding of descriptive fields at the beginning of each line in the resulting combined logfile. Lastly, logmerge can read.gz/.bz2 files.
logmerge is cross-platform and it works on Mac OS X, Windows and Linux.
Detailed instructions on how to install and use the logmerge utility on your Mac are available HERE.