well some gyms kinda do but not really im talking full on always double battle
There's a bunch of problems there, I'll just list them;
1)Both leaders must be on at the same time.
2)Most gym courses are built for one player to run through at a time.
3)Some people prefer to play on their own, so they could never challenge.
4)Somebody with a level 1000 stat could have weaker people pay them to beat the gym with them, thus ruining the economy even more.