[Wolves] Cron question

[Wayne Morris]

1)Is stuff in a cron file executed simultaneously or one by one where 
two items have the same time entry?
2)If you start to tar a bunch of files in a directory, then add files to 
directory once tar process has begun, are these files tarred or
left alone?
Background
I use a security cam with streamer to generate 1 pic per second, which I 
like to have started by cron every half an hour (rather than leave it
running permanently or have it start say once a day). That way if you 
reboot, sec cam images will resume 'soon' automatically, rather than 
having to
manually restart or wait for a longer time for cron to kick in. Also 
keeps file names short and manageable.
Also every half an hour, I use cron to tar the pics into an archive.
Problem is this changeover point, if streamer is started when tar is 
running then seem to get some frames stuck in wrong archive, as streamer 
gets a bit ahead of the
tar operation.
so should
00,30 tar job
00,30 streamer job

result in the tar operation being completed, before streamer starts , or 
not?
(if I start streamer a minute later, then I lose a minute of footage 
every half hour.

ideas?