What is best practice for unit testing multiple cases?

What is best practice for unit testing: include all the cases for the method in one unit test or break them out into separate ones?

Say I have

public int divideTwoInts(int a, int b){...}

And my default test case makes sure that

//Default Case

@Test
public testDivideTwoInts(){ 

int a = 6;
int b = 3;

int result = 2;
int expResult = divideTwoInts(a, b);

assertEquals(result, expResult);}

so far so good... but, say I want to make sure I handle division by zero - ie testing boundary and bad input cases.

Would I add it as a new unit test as in:

@Test
public testDivisionByZero_DivideTwoInts(){...}

OR as just another batch of code in my existing test?

(Also apologies if this has already been asked - coudn't find it)