Spanner...
Time for me to put the proverbial spanner in the works as I think some real misconceptions underline this scenario:
Harald Godwinson decides that William the Bastard is the greater threat and marshals in the South. When William lands, he is faced with a fresh English army and has a much harder time. Godwinson fights a series of battles against the Normans, damaging William's army heavily but ultimately is outflanked and defeated at the battle of Ashdown Forest. William now has de facto control of the South of England but in the meantime, Haardraade has landed with Harald's brother Tostig under his command. Many of the Anglo-Saxon noblemen and churchmen in the North and the Midlands declare in favour of Haardraade. He, too decides to name himself king of England and appoints Tostig Duke of Northumbria. William now has to hold the South against a fresh army of Norsemen and Englishmen.
I think that if William had landed in August (his original intention) he would have been defeated by Harold close to Pevensey (the area around Netherfield fits the bill). Harold was well aware of the threat of invasion from Normandy (it was the invasion from Norway which was the surprise) and he was basically waiting for it.
By landing at Pevensey Bay (then a good harbour but surrounded by high ground and only one road out to the north) William would have walked into a trap. Commanding the high ground, I am convinced a full-strength and ready-to-fight Saxon army would have defeated the Normans, French and Bretons. In OTL, Hastings was a "damn close-run thing" as Wellington would later describe Waterloo.
With William dead, the Normans would be forced to leave with Harold extracting from Count Eustace of Boulogne an oath never to return (which he used on Hardrada's son after Stamford Bridge). Harold then has two months before the Norwegian invasion....
The other area I have big problems with is the concept of the Saxons joining forces with Hardrada. The Saxons had been conquered by the Danes in 1016 and had been under Danish rule until 1042. William, on the other hand, was a Norman and Norman influence had been periodically strong in the reign of Edward The Confessor. Added to that the fact that Tostig was universally hated , not least by the Godwinsons, and I strongly believe that given the option the Saxons would have joined with the Normans rather than the Norwegians.
I can just about conceive of a Danelaw-style division of England between Harold Godwinson and Harald Hardrada if Godwinson is unable to defeat the Norwegians militarily. Such a Treaty would not end conflict as I suspect the Saxons would ultimately wrest control back from the Norwegians but it would take time.
The key point for the future development of England is that the Norman Conquest in OTL meant the economic and political power base of England shifted to the south and east. London would be the capital and developments across the Channel rather than the North Sea would be of greater import. This would be confirmed by William's genocidal depopulation of the North in the late 1060s which, I would argue, has ramifications to this day.
A successful Norwegian invasion or a partial success changes this. York retains its importance and in the event of a full Norwegian victory and the emergence of a Scandinavian-oriented England, Jorvik is the capital and economic wealth and importance is centred along the east coast. The south is a backwater of mainly agricultural land and countryside where the rich citizens of the north go on holiday in the 20th Century.