On all models I am aware of, both fans should come on at the same time. If one runs and the other doesn't, it almost certainly means either a bad fan or break in the wiring.
I think the reason we (myself included) obsess about coolant temps is we are used to the gauges on other cars, which are not truly accurate and tend to stay at a steady indication despite changes in ambient temp and whether AC is on, etc. MB, on the other hand, wants us to have accurate info, and thus we get freaked out when we see a needle climbing to the 100 mark.
I agree, temps up to 100 or even a bit higher are normal in hot weather with AC on, and mine does this despite having a new water pump and radiator, but reminding myself that this is OK is a daily struggle for me.