Tostig I think would be King. He might make peace with William as both William and Hardrada are of Viking heritage, essentially. However, William could defeat Hardrada but I think that he won't be King of England and the Vikings will ultimately win. just my opinion, I'm sure someone else will have a better answer.
I don't see any way in which Tostig would end up as King. Tostig threw his support behind Harald in an effort to retake his Earldom of Northumbria. Harald will be willing to give it to him (although, even then, I don't know how much Tostig will hold the position ... the Northumbrians hated him, and he had been driven out by rebellions twice before), but won't be willing to hand over the Kingship. That is going to be his.
I agree with another poster, that I could see the English rallying around Harald in order to drive out William. England already had a tradition of having a Nordic ruler, and there were some strong cultural and economic connections between the two regions.
I suspect that Harald rules in England for several years before passing away and dividing his kingdoms between his two sons.
Now, an interesting thought: Olaf III in OTL sought to foster the creation of urban centers in Norway. In the ATL, I could see him seeing the English as a model for the cities he wants to build. Perhaps this would even extent to inviting English settlers in to found cities in Norway, creating a situation where the cities of Norway are dominated by an Anglo-Saxon elite (which would certain push for further trade and political ties between the two(or one?) realms.