This HAS to be idiotic...but please explain why it is!

It is certainly an option... The biggest issue is the "shifted" ground between the charge controllers... Ground = zero volts for one controller, and the other has +24 volts as "ground". This means that typical communications and control circuits that use a "common signal ground" will not work (RS 232, RS 485, some contact open/ground outputs, one-wire bus for Wizbang Jr. comm, etc.). Others like standard cat3/cat5 Ethernet (transformer isolated), level shifting/isolating buffers (RS 232/485 buffers) can work around this issue.

People do mix old and new batteries together--And get useful life out of them--It is just a recommendation (at least on my part) that you try to avoid it--If practical.

If you need to mix old and new batteries--I would probably suggest paralleling into 2x24 volt strings rather than two 24 volt stings in series... But I am not sure that would never use an "old/good" lead acid string in series with a "new/good" lead acid string (same type of battery--model/mfg ideally).

Older lead acid batteries tend to have high self discharge. So when you put old+new in series--You have to "over charge" the new battery to make up for the self discharge of the old battery. More equalization of the new cells will use more water, erode plates, cause more positive grid corrosion.

How bad that will be--I am not sure I can tell you. Will the 2 year and new batteries "fail" (age/cycle out) in 6 years instead of 8? I cannot tell you.

Would you be happy if your bank lasted 6 years (instead of 8-2 for old bank) and 8 years for new bank?

When your bank gets ~6 years old--And one or two cells start to die every few months--When will you pull the plug on the whole bank and replace it?

There is probably enough variability in any lead acid battery bank that, in the end, you may have troubles isolating failures in 4-6 years as being caused by mixed age of batteries--Or just the luck of the draw (battery mfg) and local factors (how you charge the bank, how you cycle the bank, any mistakes you may have made, etc.).

If you need the extra capacity, install the extra charging (solar panels+charge controllers as needed) and cannot find anyone that needs your 24 volt bank for a reasonable price--Then build out your bank (24v or 48v) and just keep a good eye on specific gravity, battery voltages, etc.

Your other option--Install the 2x solar array + controllers, and keep your current bank--And just cycle it more heavily until it wears out. The replace with new bank of your needs. You may need some more genset time during bad weather--But those costs are probably comparable or less than replacing your old bank now. You have good batteries and you can deeper cycle them--As long as they get recharged relatively quickly (i.e., don't take to 25% state of charge and let sit for a week until the sun returns--that will probably sulfate them--Get them recharging the next morning with solar and/or genset--Don't need to get to 100% but get 10-30% minimum state of charge back in the battery the next day). One of the alternative methods of running a battery bank is to cycle from 50-80% state of charge--The batteries do not sulfate or show "harm" from this cycling. And get to >90% State of charge every 7 days or so.

For me, I suggest looking at the risk/rewards of decisions. Is the risk of old bank+new panels+genset run time better or worse than 1/2 old + 1/2 new bank, or all new bank (and more panels as needed).

I do not think there is a good black and white answer here.

-Bill

The "24 volt return line" (aka upper "ground" connection) would need a circuit breaker on both the floating ground and the +48 volt battery bus. The "high system" ground is now "hot" and needs an over current protective device.

-Bill