How is it in my best interest not to submit a paper to two journals simultaneously? The two journals I am considering for my paper each demand that the paper be submitted exclusively to their journal for consideration.

What do I win if I don’t keep this rule?


If I get rejected by both, I will have found out earlier that the paper is not worthy of publication.
If I get accepted by one of the journals, this is very good. I will have found out faster which of the two journals is willing to publish me.
If I get accepted by both, this is really a dream. I should probably find a reason (excuse) to withdraw the publication from one of them. 


In any case how can submitting to two journals negatively impact my reputation more than the gain by being actually accepted? (More so considering I will probably leave academia after my PhD dissertation.)

What can I lose if I don’t adhere to this rule? What sanctions, if any, can I expect?
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.