The trick for a two cups aeropress is brewing at a stronger ratio (for instance 25g in with 220g of water) and diluting it with water once it's done brewing.
I do another trick that works well enough for me, I put more coffee in than I normally do for one cup, plunge, then pull the plunge out, add more water and plunge again. Just make sure you are going into a big enough cup or use two cups.
I've never had success with brewing a concentrate using an Aeropress really. I prefer using a different method like a larger french press for multiple cups.
