The standard procedure of two independent sources is neither opinion, nor fact. It is an established procedure in professional investigations worldwide, used by law enforcement as well as by serious journalism.
Again this is just something you claim, not fact.
In the realm of logic used in the real world, information is assessed by it's probable reliability in the eyes of the investigator (or judge or jury), some things may stand on their own or require only a single source because they are obvious. Some things may need more than two sources before they are even taken into consideration. As I said above, it depends on the reliability of the information in the eyes of the investigator.
That Inmarsat data and Malaysian primary radar reports are mutually supporting, is a fallacy. Under the premises of any serious investigation, if these informations were not given independently and practically simultaneously, verifiably not knowing of each other, then that means nothing, since the later publication could have been influenced by the first. I'm not saying that was the case. Just keeping a cool rational mind.
The Inmarsat data on its own is as close to scientific fact as anything can be, the data and methods have been evaluated by known experts in the various specialty fields and there has been zero disagreement among the experts that the Inmarsat data is valid and the Inmarsat interpretation of the data is also correct.
The radar data provided by the Malaysian government in their Interim Report is also reviewed and crosschecked, this time not by scientific peers but by investigators, journalists and the general public. Malaysia has stated that their military radar saw MH-370 turn around at IGARI and this fact has been corroborated by both the Vietnamese who have stated that they saw the same turnaround on their military's (primary) radar and a Thai sighting of what can only be the same target a few minutes later approaching Koto Bharu.
The Malaysians have chosen not to reveal the exact path of the plane from Koto Bharu to their last reported sighting at a point approximately 10nm west of MEKAR, but it is not difficult to project the approximate distance traveled from Koto Bharu - and using known 777 performance data, project where 9MMRO would have been likely to turn up 47 minutes later. The last reported position given by the Malaysians in the Interim Report is at about the expected distance traveled from Koto Bharu in 47 minutes and from what is known about the flight path to that point (did not enter Thai or Indonesian airspace) that is about where and when we would expect 9MMRO to have turned up had it crossed the Malaysian peninsula and turned NW in the Strait. It could have been a bit farther north but not too much, or it would have intruded into Thai airspace. It couldn't have been much further south, or it would have raised alarms by intruding into Indonesian airspace. So somewhere around 6°30'44.24"N 96°19'34.84"E is where an examination of the flight path tells us the plane had to have been even without the Malaysian Government's Interim Report's placing it "10 mikes past MEKAR."
So we are left with a report which we can accept with a very high level of confidence fitting perfectly with another report that has been so thoroughly analyzed by experts that is stands for all practical purposes as scientific fact.
A prudent person would now accept this as fact and move on from here.