Until 1517, when Martin Luther lit the wildfire of Reformation, both places shared a common faith. In fact, it's interesting to note that what we now think of as Europe didn't think of itself as Europe, back during Renaissance days. If you had had the opportunity, at the time, to ask a European traveler in the Middle East or Africa where he hailed from, he likely would have answered "Christendom" -- regardless of whether he was from Florence or Flanders.
Beyond providing a unifying presence, the Church supplied all artists of the period with a common subject matter. The earliest beginnings of northern Renaissance art are eerily similar to the Italian Proto-Renaissance, in that each chose Christian religious stories and figures as the predominant artistic theme.
Another common factor that Italy and the rest of Europe shared during the Renaissance was the Guild system. Arising during the Middle Ages, Guilds were the best paths a man could take to learning a craft, be it painting, sculpture or making saddles. Training in any specialty was long, rigorous and comprised of sequential steps. Even after one completed a "masterpiece," and gained acceptance into a Guild, the Guild continued to keep tabs on standards and practices amongst its members.
Thanks to this self-policing policy, most of the money exchanging hands - when works of art were commissioned and paid for - went to Guild members. (As you might imagine, it was to an artist's financial benefit to belong to a Guild.) If possible, the Guild system was even more entrenched in northern Europe than it was in Italy.
After 1450, both Italy and northern Europe had access to printed materials. Though subject matter might vary from region to region, often it was the same - or similar enough to establish commonality of thought.
Finally, one significant similarity that Italy and the North shared was that each had a definite artistic "center" during the 15th century. In Italy, as previously mentioned, artists looked to the Republic of Florence for innovation and inspiration.
In the North, the artistic hub was Flanders. Flanders was a part, back then, of the Duchy of Burgundy. It had a thriving commercial city, Bruges, which (like Florence) made its money in banking and wool. Bruges had cash aplenty to spend on luxuries like art. And (again like Florence) Burgundy, on the whole, was governed by patronage-minded rulers. Where Florence had the Medici, Burgundy had Dukes. At least until the last quarter of the 15th century, that is.