I'd like to know if there are any requirements for referees to review a paper in a peer-review journal. For example, for an editor in another journal, is it OK to be a referee for the paper? I think it should be OK because, if an editor at journal A has reviewed a paper but somehow A rejected it, then it is reasonable for the same editor to be a referee for the paper at journal B because he already knows it. (Assume the editor likes the paper, as well as A and B are not in the same field and in the direct competition.)
When editors are employees of a journal, it would probably be prohibited, perhaps informally, by the employing journal. But if the editor is a volunteer, then there should be no problem, although the management of the journal might get grumpy about it.
I've assumed, of course, that the two journals see themselves as being in competition. Then it would only be a question of what "full time employee" really means.
And, an employed editor can ask a superior at the journal (their employer) for guidance; either general or specific to the paper in question.