While I have not done any heavy duty field work with DF, the way I understand how it works is that the exhaust speed is the result of what the rotor is capable of giving out. Having one boost the other won't increase the output speed if both are the same. Example if the output speed of DF #1 is 80 mph having another fan the does 80 mph behind it won't increase speed at all. However two units side by side would increase the amount of output overall but not more than 80 mph. The primary diffrence between DF and turbine is that a turbine gives continous thrust vs a max airspeed output of the DF.