Two ways to work around having a single crontab.
Put your crontab entries in files in some directory, like
~/crontab.d/thisandthat.cron, then commit any changes from those to the system crontab with:
cat ~/crontab.d/*.cron | crontab -
.cron is to avoid accidentally including backup files from editors. Note that since all the files are concatenated together, any variables set in the files will also apply to files that come later.
You could also do something similar to how
/etc/cron.hourly and friends are implemented in Debian. The main
/etc/crontab contains rules like this:
17 * * * * root cd / && run-parts --report /etc/cron.hourly
You could arrange for a single crontab entry to run multiple programs. Debian's
run-parts dir is similar to
for f in dir/* ; do "$f" ; done except that it skips odd filenames, again to avoid running backup files created by editors etc.