Well, first, the question was about replacing an AGM starting battery with a 100Ah drop in, not building a bank capable of starting an engine. Even so, using 12 280Ah cells, plus a BMS capable of 750-1000A, is a really expensive alternative to a $150 AGM. Yes, it will work, though. But holy cow, you are suggesting an 840 Ah LFP battery to do what an 80Ah AGM will do just as well.
I personally know of several installations that have foregone a separate starting battery and use LifePO4 for starting as well. I think this is an unusual outside case. They are large LFP banks, with expensive and relatively complex BMS systems. They are well designed and the owners really knew what they were doing. I don't generally recommend giving up a separate staring battery, and where the starting battery is separate, I don't recommend LFP.
This is a staring battery, it sits fully charged, not because it *has* to, but because that is what it does. The engine starts, the alternator then fully charges the battery (which happens quickly, as starting doesn't use very many Ah), and it sits there until needed for starting again. If it was a dual purpose starting and house bank, that wouldn't be the case, but that also wasn't what the question was about. It was specifically about a starting battery.
How do you know the voltage difference isn't great? What is to stop someone from connecting the two banks when one bank runs dead, the BMS protects it at 10.5 volts, and the other is at 14+ volts? That actually seems a likely scenario, "oops, all my stuff just turned off, I better connect the second battery!"
I stand by my answer, LFP is a poor choice for a starting battery.
If you are answering a specific question outside of the OP then it would be beneficial to the rest of us who are engaging for you to actually quote that question in your post so we know what it is you are talking about.
Now that you have given me enough information to understand what you are talking about, lets get on the same page.
Im not a proponent of lifepo4 for a "starter battery". I dont think it is a good idea from either cost or function to replace a LA battery with a 100ah drop in lifepo4. Not smart from several perspectives. BMS turns off charging and destroys your alternator, and, holding the battery at a full SOC are the two biggest issues IMO. Never mind that the starting current far exceeds the current a 100ah lifepo4 battery can repeatedly supply.
So, engine electrical system needs something in the system to save the alternator. The only practical and cost effective solution I am aware of is to leave a lead acid battery in that system,
but, it does not have to be a standard size LA battery, it doesn't even have to be big enough to start the engine. It just has to be big enough to absorb the field windings collapsing. The vast majority of the energy to start the engine can be provided by the house batteries as described in the post you quoted above. Yes, of course the house battery has to be big enough to supply the current with out exceeding the max current ratings of the battery so that is a limitation.
You appear to have thought I am suggesting that a person should replace the lead acid starting battery with a drop lifepo4 battery capable of starting the engine. I agree, that wouldnt be a smart idea and neither would continuing to think that that is what I was suggesting.
This idea seems to be something that people have a hard time getting their heads around. Many of us have a house battery that is big enough to supply the current to start an engine. It is not complicated to integrate the house battery into the vehicle electrical system to provide starting current so you can
all but eliminate the LA battery from the system while providing isolation for the the alternator and provide it protection from the issues associated with trying to charge lithium batteries.
Lets be clear. In what I have suggested, the house battery is the "starting battery" . It will not be kept at full voltage by the alternator as it will be isolated from the vehicle system by a DC-DC or some other means. The LA battery will be kept at electrical system voltage and is there for the sole purpose of protecting the alternator. It is no longer the "starting battery" as it is simply too small to do the job.
So, as long as the house battery is big enough to provide the current, lifepo4 makes a fantastic "starter battery".
As for connecting two batteries at different SOC: