Absolutely they can date and I would go a step further and say our increased fear of fraternization the last 8-10 years has impeded knowing our troops as well as we are expected to. Now would I date someone in my organization in the military, absolutely not. Every relationship you have ends badly except for the person you marry and then those sometime as well. Quite a few in the organization are going to know their business, especially if it is a 'slow news day.' One of the people need to be prepared to find work elsewhere if it goes bad. Not saying this will happen but it is something to consider.
As long as there is no authoritative conflict of interest (the e4 doesn't directly report to the e5), yes they can date. Should they though? I would strongly discourage it because if the relationship fails, problems in the workplace could then arise. They may feel awkward around each other, not communicate as necessary to do their job, retaliate by tampering with accessible information regarding the other person, etc.
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