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Generally speaking, every person should only have one grad staff job on a site. Sites where people have more than one job often have difficulties to fill all staff positions which is why some people help out by taking on another job. However, those are exceptions and not the standard, so there is no real need to make an overall rule about it.

This kind of information should be included in the rules and guidelines part of the site. As soon as you fulfill the requirements to get the online statuses you get an automatic explanation on how to enable it with it.