O2 sensor codes can be among the most difficult to diagnose, because anything that can effect the air/fuel mixture can cause the code.
In your case, you have codes for both sides, so the fault is probably something that will effect both sides, rather than being specific to one side of the engine.
Having replaced the O2 sensors, it seems fair to assume that they are good.
Beyond that, I'm not entirely certain how I would proceed. A few thoughts:
1) From other considerations (exhaust condition, spark plug condition) is the engine running rich or lean?
2) Check fuel pressure and the fuel system to make sure the fuel system is working as it should.
3) Make sure the MAF is clean and working.
4) Be sure to check for vacuum leaks.
