If you're using TIME_PREFIX , it's just a regex, so you should be able to build in an "or" condition, e.g.:
TIME_PREFIX=(prefix1)|(prefix2)
and make sure that MAX_TIMESTAMP_LOOKAHEAD is set high enough to get through the longer longer messages.
Alternately, if the netscreen events are coming in via syslog, you could have the syslog server write them to two separate files (based on pattern match, syslog facility, or whatever works), and then create two distinct sourcetypes within Splunk.
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