Which step in the budget process do both houses of Congress control?

Both houses work on spending bills based on the president's budget.

Explanation:

The House of Representatives and the Senate are both authorized to discuss over the spending bills once the President presents the budget for the given financial year. These bills include the provisions done for the expenditure to be done on the functioning of the government and on the implementation of various schemes introduced newly. The houses reach on a common ground before the bills are worked on and passed.