Yes, drag flaps would work nicely IF there's a differential. I suspect that is not in this case because of the chosen method of yaw control. Searching the net, I see most amateur coax designer/builders use this method for equalized counter-rotation.
I can't get over how small the diameter is for both rotors.
THOSE WERE TEST SUBJECTS ONLY NO PRODUCTION
Cicare had his designs taken, He's a great designer.
As for the coaxal stuff they mostlyall use standard collective & cyclic controls - Yaw is reversed unless a special null cam is used or tip break servos.
Fixed pitch with mast tilting in the cheaper 'kits' are falling bowling balls with a seat when the engine stops.
All a coax does is add to the costs of extra main rotor blades - controls & headaches - The standard single main & tail systems do better.
I'll take a tandem Chinook over a Kamov coax any day. and a standard Hughes twice on the Weekend.