Initially,what I understood was two edf's , one in front of the other, and if they are counter rotating then the rear unit would be trying to change the direction of the airflow from the one in front. And the rear edf would be under a tremendous load trying to utilize the airflow from the front unit and changing airflow direction at the same time. I dunno, maybe the only way to prove this is to put these two counter rotating edfs in a common tube as suggested and crank them up and monitor volts and amps of both, one at a time then simultaneously. I think the only way to keep from being totally baffled by all of the answers you'll get, is just to experiment on your own and post the results. Think out of the box..
Yup, I agree, build it, crank it up and see what happens.
Not all that happy with some of my EDFs using 4-blade impellers. What I usually do is immediately discard the 4-blade impeller and replace it with one that has more blades like to an 8, 10, or 12-blade impeller so I have several 4-blade impellers laying around collecting dust.
Tandem 4-blade might be an interesting test even though it won't counter rotate.
First test with standard single impeller for comparison and actually I already have that data from an Eflite EDF using a 4-blade impeller.
Now all I need to do is try the two identical 4-blade EDF impellers glued together with epoxy with 1st and 2nd impeller blades offset approximately 45 degrees from each other.
Doubling up two separate impellers on the same shaft should increase thrust but also current draw and should give different thrust/current readings from the standard 8,10 and 12-blade impellers whose data I already have from previous testing.
Probably need to Change the shaft length in the motor so the tandem impellers fit, then slap it all back in the EDF housing and give it a go.
Turnigy Thrust Stand will measure max thrust, current, watts etc.
The only thing the Thrust Stand won't do is put out the electrical fire from the excessive current draw.
And on the 8th day God said: "Go NAVY",.. and it was good! Pray like everything depends on God. Prepare like everything depends on you.