Rsyslog filter

Ubuntu uses rsyslog so you can configure it to filter messages and write them to arbitary log files e.g create a file /etc/rsyslog.d/shorewall.conf

:msg, contains, "Shorewall:" /var/log/shorewall
& ~

Or if you want to split things out some more

:msg, contains, "Shorewall:cust2net:ACCEPT:" /var/log/shorewallc2na.log
& ~

:msg, contains, "Shorewall:" /var/log/shorewall
& ~

Which puts all messages containing "Shorewall:cust2net:ACCEPT:" into one log file and any other messages containing "Shorewall" into a different log file.
You need to restart rsyslogd for this to take effect.