Check out this link:
https://www.npr.org/2016/01/31/465016222/how-predictive-are-iowa-and-new-hampshire
It shows who won Iowa and New Hampshire, both Democrats and Republicans since 1976. The winners are shown in two, easy to read images.
Notice that the following people won both Iowa and New Hampshire since 1976 and they all went on to win the nomination. Fifty percent of the time, they also went on to win the General Election:
1976: Carter won both--Got Nomination--Won General
1980: Carter won both--Got Nomination--Lost General
1996: Clinton won both--Got Nomination--Won General
2000: Gore won both--Got Nomination--Lost General
2004: Kerry won both--Got Nomination--Lost General
2012: Obama won both--Got Nomination--Won General
The following people won either Iowa or New Hampshire and then got the nomination. One out of three times, they went on to win the General Election:
1984: Mondale won in Iowa--Got Nomination--Lost General
1988: Dukakis won in 1988--Got Nomination--Lost General
2008: Obama won in Iowa--Got Nomination--Won General
Since 1976, the following people won neither Iowa or New Hampshire but still went on to win the Nomination and the General Election:
1992: Bill Clinton lost both--Got Nomination--Won General
So, what does that tell me? Based on the past, if anyone wins both Iowa and New Hampshire, their odds of winning the Nomination are quite good. If, however, two different people win Iowa and New Hampshire, The odds of one of those two people *combined* winning the nomination is probably in about the 75% to 80% range. This means that each of the two winners' individual chances of winning the nomination might be around 40%.
Here's where it gets tricky. If those two winners are facing someone who is strong in the South, like Joe Biden is, and if that person sweeps the South, that person can override their two wins. Just as Bill Clinton was the Southern exception in 1992, Joe Biden could be the Southern exception in 2020. It means that if Joe Biden does not win Iowa or New Hamshire, he still has a shot at winning, albeit maybe about a 20% chance of winning. However, if he does really well in the South, as could happen, he could very well do it.