What does this mean? End of the Viking Age? England rule from Norway or Norway ruled from England? England given to another son when the king dies? Will William of Normandy still invade England and if so, what are the chances he beats Harald?
@Nik
That's debatable. Some earlier historians argued that the two of them coordinated their invasions, but as William sat around waiting for good winds for some time the general consensus now is that they were acting independently. The idea of William and Hardrada being in cahoots was (is?) popular with those who argued for the Norman conquest being a bad thing overall, so wanting William to be not only a 'grasping invader' but also untrustworthy in that, according to this theory, he would just have attacked the weakened Hardrada anyway.
Of course, we'll never know for sure...