How do Split SSA -1099

The best way is by number of days in each location. For example, you were in State A for 300 days and your benefit amount is $10,000. $10,000/366 days gives you $27.32 per day. $8,196 would be allocated to State A.

if you are a part year resident TurboTax would do this calculation for you.

At least, it should. That's what you're paying for.