Are you familiar with the term "fiscal year", and how it applies to the federal budget?
If so, then let's turn our attention to revenues. While the Bush budget envisioned a deficit of $400 billion, revenues fell $600 billion short of projections due to the Bush recession, which, unless you want to blame that on Obama, means that the actual deficit of the Bush budget, as proposed, was $1,000 billion.
If you want to account the additional $400 billion in spending over and above what Bush originally proposed, to Obama, then that would make
Obama 'responsible' for $400 billion of the 2009 deficit and
Bush 'responsible' for $1,000 billion of the 2009 deficit (double the $450 billion deficit the Bush budget rang up in 2008, which was actually $962 billion if you measure the increase in total outstanding debt for FY 2008 -- $9.06 trillion to $10.02 trillion).