not quite
cone area of 2 18s = 508.94
cone area of 1 24" = 452.16
Its just like comparing 2 12s vs 1 15. Almost the same but not really, saves on a lot of money and weight though.
Honestly, with that much cone area, your gonna get a nice hit with metal regardless... unless the box is really oversized tuned to 28 hz and under or something.
a good tuning point for you should be around 35-38hz with a 11-12 cubic foot box if EDM, dubstep, metal and rock are your priorities. If your hip hop library is quite large, go with 30-33hz
33-34 is a good medium most of the times. leaves things relatively flat but that depends on how well the driver models in box designing software.