Bran

There are some inverters with dual battery input, for example certain Deye models can do that. But in your situation it does not make any sense to go this way. One of the advantages of your current 48V system is, that you can add more batteries in parallel as you go, without any hassle.

Hi @Bran
I assume that the batteries are something like "SOL-B-L-M200 Mecer Second Life LIFEPO4 Lithium Battery 12.8V 200A" and you have 4 of them connected in series. So it's 10kWh of storage.
Do I get it right?
If yes, then:
These SOL-B-L-M200 are LFP 4S for 12V, 16S for 48V. BMS is just basic, nothing super-complicated and works without any communication with the inverter.
So, just get any LFP battery that will be 16S for 48V, parallel it together with these four SOL-B-L-M200's and you're OK.
The best would be to charge your existing battery to 100%, then disconnect and charge the new battery to 100%, then connect together.
The new battery should be at least 100Ah x 48V (16S). Reason is, that if M200's BMS will suddenly disconnect, all the charge/discharge load of Axpert 5K will be directed to this new battery. Therefore, the new battery must be able to handle such a load. Adding 200Ah would be even better, of course.

You did not asked, but I have to react anyway:
East+north+west is great for feeding the grid, for PV without batteries, PV with flooded-lead-acid battery (FLA charges really slow at 90%+ SOC) and/or PV with a very small battery.
In all of the above cases, designers prefer to stretch the production hours, because the small, weak, or non-existent battery is unable to store all the energy produced during the noon peak. Therefore, it's better to sacrifice the peak and spread the PV generation over longer hours.
This E+N+W arrangement works excellent over spring, summer, fall, but has very poor results in winter.
For a serious offgrid system in SA, with a large lithium battery, the best is to point all the panels to the north.
Reason is that in the serious offgrid setup the spring, summer and fall are of no issue, but the winter is where you want to squeeze every watt of PV production, in order to survive.
For offgrid that's designed to work during the winter it's battery bank has to be so large that it will be capable of running the loads for a number of dark days in a row.
Therefore, it's best to squeeze all the winter PV generation (where north is the best) and store it in the battery in order to use it tomorrow...or day after tomorrow.
I am not pushing you into rearranging your panels, just want you to know that if you will gradually increase your battery to something like 50kWh or more, then the E+N+W orientation of panels will start to be the winter bottleneck.

This is a Grey area as to why supplier doesn't permit parallel connection. It could be due to bms compatibility or due to the fact Mercer use pouch cells in these specific models or due to warranty issues( basically play it safe approach.) @Scorp007 If you have used them in parallel to the same cell count same chemistry lfp battery for 3 years without problems then maybe the Op can use a diffrent lfp battery in parallel. It would be much better $$$ wise to just add another 5kwh battery than 2 inverters with separate batteries.

My 1st question would be are the batteries still under warranty?
If not I would parallel the Mecer with any other lithium that uses phosphate cell or even another set of Mecer. I do take into account the cost from @TaliaB and I base it on the fact that I have used my Mecer 200Ah in parallel with 2 other totally different batteries for 3 yrs. No problem. I do THINK the reason for not using the Mecer in parallel could be that during a specific fault condition the Mecer BMS might be at risk. Here I call on @TaliaB to share with us his insight.
I would just ensure that each bank must have their own suitably sized fuses and the main fuses between batteries and inverter.
I have to point out that although my Mecer could provide enough current in their early life they later still had a good capacity but had a high voltage sag during starting of a fridge. This I would take into account that one might not get the 5+ yrs of service prior to buying a 2nd bank of Mecer but look at another brand. My Hubble S-100 are performing much better under fridge high surge current after 4 yrs.

Yes the Mercer bms is not capable to handle parallel setup. You can run 2 seperate inverters by means of splitting your loads that is isolated from each other ( seperate live and neutrals). If you don't have space for more solar panels then also split your solar array between the 2 inverters 2S3P. For the second inverter you can then decide on different lfp battery pack. What is important is that each inverter has seperate loads connected that is completely isolated from each other as the 2 inverter outputs can not synchronize.