How is it in my best interest not to submit a paper to two journals simultaneously?
It is quite likely that there is an overlap in who gets asked to referee the two submissions, which would lead to the double submission being detected.
Double submission of papers is sufficient ground for retraction - even after the paper has been accepted for publication. As some journals publish submission dates with published papers, it is conceivable that your double submission is uncovered after publication - and that you end up with a retraction, ie no published paper.
Academic misconduct (and double submission is such) typically is sufficient for dismissal from a PhD program.