I am a bit confused at what you are trying to do, as it sounds like you are using Gliclazide like a bolus insulin, to achieve an immediate effect, rather than as a bolus one to have a continuous one? And the level of reduction gained is somewhat meaningless in isolation.
Normally if taking 160 mg you would split it between the morning and evening meal, as its half-life is up to twelve hours so that way gives you a constant benefit. So using it to receive an immediate burst of insulin would not work.
Also, when you are fasting and you body is in good balance, the pancreas is still producing insulin that it stores ready to release immediately when you eat. So you do not just want the benefits when eating. And Gliclazide is particularly good at helping this first phase of insulin released compared to other similar medications, which also reduces (though does not eliminate) the risk of hypos from just over-producing insulin.
So if you take both tablets together then you will have a better response for half the day, and a poorer one of the other half compared to splitting the dose. And depending on what you are doing and eating in that half of the day, you may not be gaining any benefit if you are not needing that insulin and instead you body is just having to release more glucose to keep it under control.
But I am no expert either. If you read the patient information leaflet it should tell you that if you need more than one tablet a day they should be split like this rather than taken together. It is why if only on 80 mg it will normally be as two 40 mg tablets rather than one 80 mg one. Though as always this is in general and there may be individual reasons to take a single dose.
When I was diagnosed I was put on 40 mg Gliclazide twice a day. On the first day my morning and evening fasting levels were 10.3 and 10.6 mmol/L. Around a month later they were constantly around 5 and 6 respectively. Unfortunately I was not told to test after a hypo, so my notes make reference to many that were probably false ones as my body was not used to being that low.
That was enough to get me from 11.2 % / 99 mmol/mol on diagnosis to 5.0% / 31 a year later, with no real lifestyle changes and it was not particular bad to begin with. Ironically when I was diagnosed I had been thinking I really need to drink less pop (and I only drank diet versions) and more fruit juice, so that never happened.
Apparently within that first year I hypoed for six days in a row, eight days in eleven, so stopped taking Gliclazide. Which lasted for just over a month because my evening fasting levels rose to around 10 mmol/L so I restarted it.
I have no memory of that, but what makes it interesting is that after taking my last tablet on a Monday evening, I felt shaky with a 4.2 the following day. Then on Wednesday hypoed with a 2.9, and also hypoed on Thursday, Friday, and Saturday. So I think that suggest something about how Gliclazide builds up in your system that it takes about a week to clear, and therefore why it is not helpful to think of it as having an instant effect but instead as keeping your system topped up.
