If it was any other state than NY, you would claim 2/3 of income from NJ, and 1/3 income from NY.
But New York (and Pennsylvania, Nebraska, Delaware and New Jersey) taxes telecommuters, so it's all taxable by NY.
Reference: https://taxnews.ey.com/news/2020-1411-new-york-senate-[product%20key%20removed]-businesses-to-treat-...
Which states (at New york): The Department of Taxation and Finance posted on
its website the following response to FAQs: “My
primary office is inside New York State, but I am
telecommuting from outside of the state due to the
COVID-19 pandemic. Do I owe New York taxes on
the income I earn while telecommuting?
If you are a nonresident whose primary office is in
New York State, your days telecommuting during
the pandemic are considered days worked in the
state unless your employer has established a bona
fide employer office at your telecommuting location.
There are a number of factors that determine
whether your employer has established a bona fide
employer office at your telecommuting location. In
general, unless your employer specifically acted to
establish a bona fide employer office at your
telecommuting location, you will continue to owe
New York State income tax on income earned while
telecommuting.”
