To be in accord is to be in agreement. The noun usually refers to a state of harmony between two or more people or groups. To be in accordance is to be in compliance. For example, you might build a house in accordance with zoning laws. Accord usually takes the preposition with, while accordance can take either to or with.
While accordance has only the one narrow definition, accord doubles as a verb meaning (1) to cause to agree, (2) to bestow upon, or (3) to be in agreement. It’s also a noun meaning (1) spontaneous desire to take action, or (2) a settlement between conflicting sides. This last definition is the most common sense of accord found in news sources.
In accordance with and in accordance to could often be shortened to according to.
Accord is a synonym of agreement or harmony—for example:
Until a couple of days ago, I was in accord with those who maintained that Trump would never formally announce his Presidential candidacy. [New Yorker]
This view is not in accord with reality. [Globe and Mail]
The presidents of Chad and Sudan sign an accord in Senegal aimed at halting five years of hostilities between the two countries. [BBC News]
And these writers correctly use accordance as a synonym of compliance:
However other aspects of the pursuits were found to have not been conducted in accordance with police policies. [New Zealand Herald]
Terrorist ideologue Osama bin Laden was buried at sea in accordance with Islamic custom … [Wall Street Journal]
It was sunk in accordance with the recognised rules of war. [Telegraph]