It is just a way to gain more time since a journal would take a long time (about several months to make a decision) and I intend to withdraw my manuscript from one of them when I get acceptance from the other.
One often hear about that it is discovered because both journals send the manuscript for review with the same reviewer. Often the subject makes it obviously to select the same person for the editors of the two journals.
All real journals have the condition that a manuscript is submitted on the conditions that it is not considered in another journal.
If you get caught doing this the punishment I have read about is that the authors(s) are banned from the publishers journals for a period.
2) Doing what you propose just slows things down further if you purposely waste referees' efforts.
As a referee, I would be annoyed.
I know that journals often take too long, are arbitrary, and may be unfair to the authors. Fortunately I think that there may be pressure to speed this along now that there are so many more outlets for your work, which people may find on the internet without dealing with the old system of publication at all.
If you are in a hurry, you might chose a faster outlet instead, rather than a (currently) more 'prestigious' journal. But be wary of "predatory journals."
What Dr. Henrik Rasmus Andersen mentioned that may happen.
But the basic thing is honesty. This type of practice (submission of a manuscript simultaneously in two journals) should be avoided. Rather, you may follow the suggestion (of Dr. James R Knaub) for faster publication. Generally, good journals take time to publish.