Because more than one hosts does the dumps of the 'regular' (not en wikipedia) wikis, if one host is still running a previous dump run but the other has completed it, we can have problems. Specifically, one host will prepare all unlocked wikis for the new run, but it won't be able to prepare the wikis with dumps in progress. The second host, when it finally completes its jobs from the previous dump run, will run the cron job and try to prepare the wikis again with a different start date.
If the pervious jobs have all completed before the next run starts, this is of course not a problem.