Hi, I am filing joint state return for NJ & NY, we are part year residents of both and have non-resident income from Oregon, how do we split it between NJ and NY ?

You will allocate the Oregon income based on where you were a resident when she earned it.  You can calculate the amount (or look on your pay statements) or use a % based on time spend (75% to New York and 25% to New Jersey).

@MaryK4 , thanks a lot and since we paid taxes in Oregon, we would divide the taxes paid in the same way to take credit for taxes paid to another state ? (To avoid double taxation)

Yes. Split your nonresident Oregon tax liability using the same proportions as you split your income between New York and New Jersey.