but what if the one in the back was not turning the same speed but much faster? Say you used a 2000Kv motor in the front and a 4000Kv in the back. It might work since the front one is effectively reducing the load by feeding the air to the rear one. It might be better for speed than side by side since the crossection would be smaller and the outlet could be kept larger thus keeping the thrust up but boosting the eflux speed.
Sounds like a fun experiment.