We won't know our financial situation in two year, we can only forecast it. It's possible we'll pay for legislation we have no money for. The reason a budget is done by the year and not two is so we know. In fact I would even support a smaller time frame for budgets, and specific criteria for each of these budgets throughout the year. The programs could later be revised to shift money to other programs, or entirely repealed when the next "specific" budget voting comes into play. For example a budget for military, or budget for social programs, budget for scientific programs/departments.