The tests should be two hours apart - it's setting a baseline and then looking for the effect. I'd advise sticking to one pattern - eg test immediately before eating and then two hours after the first test. It just cuts down on the number of variables.
The point of testing is to see how well your system deals with whatever you ate. Rises in blood glucose will typically occur within a few minutes of eating carbs, peak in the first hour after consuming, and should be back at or near baseline by around two hours. That's the way it should happen, and you're testing to see how well your system does it.
You can of course test any way you want, and in the past I've done tests every 30 minutes after eating once or twice just to see what was happening - that was a while ago though. If you want to see that sort of information, CGMs are really good at showing the pattern of blood glucose changes across a day, and not just in response to food. The CGM graph I've attached is from a non-diabetic person, and shows a lot of variation. You can probably easily spot where the meals were.
However, it's not an exact science. I've had meals that lasted five minutes, and meals that lasted five hours. You'd obviously test differently for those, maybe several times.
