My opinion: Solo both portals at the same time. Maybe send an extra in for a few seconds to fire off a few rockets then leave if the two soloers aren't 100% competent at soloing. This is easily the quickest most efficient method IMO.
These are the 3 main strategies ive seen used with only mild variations, so if you use a method similar to one of the above vote for that one.
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One guy in each portal [i]simultaneously[/i] Each call out low health, then kill their gatekeepers and grab their relics This is better than one at a time because it prevents you and your team for sending someone or a few people to the middle to fight off the never ending spawns of Minotaurs This method is [i]extremely[/i] helpful on hard mode