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Dont forget to check the o ring on the dipstick too.
Low rail pressure will cause the same readings too.
I would lean toward intake manifold leak. Gaskets or hoses that go to it.
What i would do is set it up to see live data on stft and spray small controlled squirts of starting fluid along the seal areas of the intake manifold. If you get a leak there you will see the fuel trim flip really quick to negative numbers. Then you will have an idea of approximate leaks. Then switch to a spray bottle of water. Spray in that general area and if you get water on the leak it will make a slurp sound.
With both sides doing the same its really unlikely to say it is exhaust related (not impossible). Look at parts that are common between the two banks. Intake gaskets are specific per side but they share a central plenum, throttle body gasket, brake booster, map sensor and o ring, purge solenoid (very common with gm), intake tube hole or not attached correctly, catch can hoses.
Usually you should see +5/-5 fuel trim at idle. So just check what you can until you narrow it down.
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