they wrote the software that selects the maps..
its probably 1 or 2 extra lines of code to prevent one arena from using the same map that is already up in the other..
Eh, a bit more, but not by much...
Take C# for example, all they'd have to do (assuming they have it set up so that each map has a random number [let's call it rnd], and when it's time to switch they have it pull that random number) they'd need this...
do
{
map1 != map2;
break;
map1[rnd];
}while[true]
Meaning, if map 1 is not equal to map 2, then break out and go on normally, otherwise, new random. And it will keep running this do while loop until it's "true." So it will do this until map 1 != (not equal) to map 2...
And for the other arena just switch the variables.