It is an oft repeated line and it does have some basis in truth in Christian missionary movements in general, but it not extensively attested in primary sources from the time period you're asking about.

I'm going to split this response in two two general parts. One that tackles the Norse experience of religion as well as we can reconstruct it, and then actual examples of this sort of syncretism.

/u/textandtrowel can come by and tell us all about the the possible christian communities in Scandinavia that may have existed before the "official" conversion of these societies. I'm going to focus more on the textual evidence (limited as it is). He might have a good bit to add for the Norse in Ireland and material evidence.

Unfortunately for us, a great deal of the writing about Scandinavia's conversion to Christianity is unreliable. There are the sagas and hagiographies that were written after the conversion period, but these sources are clearly biased and cannot be fully relied on a total and comprehensive account of the events.

Iceland is a useful case study however. Early on in its history it allowed for broader religious toleration for some time before it became thoroughly Christianized. Again we do need to keep in mind issues about authorship however.

These laws do not really make a syncretic distinction, instead describing Christian and Heathens, not any group of people that cut across these lines.

Unreliable as they are, the sagas might also be of some interest here as well. The saga of Haakkon the Good comes to mind. He was supposedly fostered in the court of the King of England and adopted Christianity before returning home. However once he arrives there he faces stiff resistance from the heathen court that he has come to. He flatly refuses to moderate his stance and refuses to take part in the heathen sacrifices and rituals. A bystander suggests he "eat" the horse meat through a rag so as not to actually consume it and yet still eat it, but nobody is happy with this.

Other figures involved heavily in conversion such as St. Olaf also refused to make such equivocations in their fervent belief. He famously persecuted heathens and destroyed pagan shrines. However, as cannot be understated, these sources are not entirely reliable. St. Olaf's own involvement is under heavy scrutiny, especially given the hagiographic accounts that had a vested interest in playing up his actions.

This also comes through on more contemporary sources that were arrayed against the Norse. Asser's biography of Alfred the Great makes frequent invocation of the religious dynamic between the Norse and the Anglo-Saxons. Conversion was a pre-condition for peace between Guthrum and Alfred, and it is unlikely to me that Alfred would have overlooked Guthrum remaining pagan but paying lipservice to Christ. This is admittedly my own speculation, but I think it is borne out by the facts on the ground. The Norse in England took to Christianity relatively rapidly and maintaining paganism, even allowing for Christ's place in the pantheon, would not have been tenable.

However despite all the lack of evidence for extensive syncretism in the Norse world, this trope did not come from nowhere.

The famous instance of Germanic paganism and Christianity existing side by side for a time and even syncretic practices forming, comes from Bede's account of the British king Rædwald of East Anglia. According to Bede he maintained a temple with shrines both to the Old Gods and to Christ in a relatively impressive structure.

However this period of syncretism did not last, and the pagans and Christians rarely seemed to co-exist long in Anglo-Saxon England. Bede is our main source for these events, but again he is coming to this from a definitively Christian context (he was a monk and Christian intellectual) and cannot be relied on entirely.

The direct textual evidence availible to us indicates that there were not widespread attempts at syncretism between Christ and the Norse pantheon. Syncretic elements DID absolutely show up in the religious life of these people after conversion. However, the presence of Christ as an element of the Germanic pantheon seems to be pretty unsupported.