From someone who stays in the onexone section, I believe that most people who say that in that in the onexone section are wanting to create two pairs in one plot. Or that is at least the case from my own terms of the word and many others I role play with. You must play one character in each gender so (if both writers are female) you both get that chance at playing your own gender. I know for some cases it's also a way to weed out people who are not willing to be very flexible with their writing (Only one character[wont play NPC] and one gender, wont help push a plot a long and such). It's currently becoming a bit of a craze in the onexone section, since it is very highly populated with female writers (or those are the writers who mainly post interest checks). There are some people out there who love playing a gender that is not their own, but there are many who are willing to play the other gender but always prefer to play their own. It's also away to keep the interest going longer if at one point one pair is moving a little slowly but the other isn't you must keep posting with both pairs to keep the story going and thus push through that slow part before things pick up again. but those are just my thoughts.