I contacted Hanna to get their take. They recommend the 2 cuvettes approach for those who have trouble preparing the sample before the automatic shut-off. The guy I talked to said the difference in the glass with the two cuvettes would cause very little difference in the final reading. He said the important thing is the two minute mixing time followed by the 3 minutes in the checker. He said the whole test is geared around those 5 minutes. He also said to invert or lightly shake the cuvette during the 2 minute mixing time (e.g. don't shake hard/fast).
The 2 cuvettes approach eliminates the time issue for me. Also, even when I have completed the test, I have never shook the test sample for more than maybe 1:10 and haven't had time to thoroughly analyze for bubbles when shaking this long. After talking to this guy, I think I'm better off getting everything else exactly right and accepting the fact that using both my cuvettes might insert a very small margin of error.
