Your observation about possible 50% loss of suction via the second chamber orifices being open led me to perform a test both ways.
With both chambers open there was a slight loss of suction, however drill shavings were completely removed from the work surface.
With the unused chamber sealed off with tape a stronger pull of air could be felt at the infeed air opening. Obviously, this was not a scientific test with instrumentation. To err on the safe side & eliminate the possibility of unremoved debris fouling the 20 mm boring bit, I decided to augment my DES (Dust Extraction Shoe) with a 90 degree closure segment affixed with rare earth magnets.
Many thanks for your alerting me to the possibility of reduced suction. With your single chamber device, it precluded the loss of suction issue. You obviously had that in mind when designing your unit. See attached photos.