There isn't a pre-defined relationship between voltage and trim... You've seen yourself that voltage can fluctuate the same on both banks while trims are different. It's unlikely one of the new lambda probes is at fault or one less compatible than the other if they're both the same spec.
The engine has a trim for each bank specifically to allow a different trim on each bank, there would be little point in having a different trim for each bank if trims were supposed to be exactly the same. That said, we usually expect trims to be similar - Trims are there to compensate for tolerances in sensor readings and mechanical aspects such as fuel pressure, injector flow rates, valve train (effecting airflow), even to compensate for the ECU's fuelling map not being quite correct. Some tolerances change over time as mechanical components and sensors age/wear. If everything was made to an exacting standard and remained the same as it ages trims wouldn't be necessary. We expect trims to be similar because tolerances such as fuel pressure and MAF readings will equally effect both banks of cylinders, wear on valve gear should be about equal, good injectors should flow almost equally. A small difference in trims isn't unusual, a large difference in trims could point to something being different between banks, since bank lambda readings are the average of the 4 cylinders on the bank something effecting just one cylinder can cause a bank trim difference - a leaky petrol injector on one cylinder might cause it's bank readings to have more negative fuel trim (rich), worn valve gear might cause a cylinder to pump less air resulting in the same more negative fuel trim (rich), a vacuum leak on one side of the engine might cause positive fuel trim on that side (lean).
The expected / normal fuel trim isn't necessarily zero, an ECU's base map (with zero trims) may intentionally be set to provide slightly lean or rich mixture before fuel trims are learned (even if the engine, sensors and fuel system components are all exactly in the middle of tolerances, bang on spec) but the ECU is always expected to provide correct mixture when fuel trims have been learned.
Before changing the exhaust it would start OK from cold but had problems restarting when warm... Now we're talking about trims and poor idle but no mention of if it starts ok when warm now. Anything else to note after changing the exhaust/sensors? If the engine seems good and drives/idles well enough, if trims aren't miles apart I would drive it for a while and see if the idle improves / trims come more in line / restarting when warm is any different.