Can I safely add a 12V 200 AH AGM battery to a 12V 230 AH AGM battery (from different manufacturers) for a small solar system?
No, you should generally only combine (connect in parallel) batteries that are identical, have the same age, are used in the same way or better: are new.
Also their voltages should be nearly the same, if not one battery will charge the other which can have severe consequences (too high currents flowing).
Batteries from different manufacturers, even if they have the same chemistry, should not be combined as it can result in unforeseen effects.
If both lead acid batteries have similar chemistry (AGM) and not over-stressed, I see no problems.
Here I've shown a simple model of two batteries. Each cell has two time constants that can be derived from ESR Ah with the Peukart constant for the each battery:
But rather than prove that, let me say each T constant can be modeled as follows; - the short term recovery time to step load being removed. - the long term recovery time to being loaded heavily for several minutes.
Now putting two pairs of RC circuits in parallel does not pose any problems. This is why we can jump start cars with lead-acid batteries without risk, unlike Li-Ion cells which have a PTC voltage effect and can lead to excessive circulation currents.
The model I have used uses the cell voltage of 0% SoC although we know that ESR also rises sharply at 0% SoC, I did not show. So if a weaker battery depletes or decays under load much faster, it contributes much less current without being a burden to the other battery. As long as the cell voltages shared stay in a healthy range, it operates as if it is single battery even though in reality it would be a 6S2P array.