need 24v use 2 12v?

Newcomer here. Just joined 1/7 at 10:25 AM. See that post for details on what I am up to. In short, I am replacing 2 12v SLA batts in series with an some 38120 3.2v cells in 8s1P config. I looked around for a 24V 8s BMS but the only ones I could find were really expensive. Then yesterday I was looking over the various forums and threads and came across a 24v discussion and had some examples of 24v 8s BMS from Alliexpress that looks just like one I found on fleabay or Amazon or somewhere for less than half the price.
So question is: Can I use 2 12v 4s BMS? essentially use two Li-ion banks of 12v each just like the old SLA?
If I go with a 24v8sBMS how many Amps do I need? cells are 8Ah each. Separate or common? (see
)
They have BMS from 18A to 60A

Can I still use the old charger? It is 24 nominal but is putting out 27.7v at 1500mA , It was a very slow charger with the SLA batteries and won't get any better with a new pack of LiFePO4

only you can answer this type of question.
Capacity depends of maximum instant load and time. So you need to make sure your BMS will support it.
Actually, electric starters can gobble up a lot of Amps. so if you have a 17Amps SLA, you can expect that the required current is probably about 30-40A.
This would probably kill an 8 amps lithium, unless the C discharge rate is indicated (you would need at least a 5C).
so to be safe i woudl build an equivalent of the SLA (2P8S) , this woudl give you a 16Amp 24v Battery representing a lot more of energy than the SAL, because you can almost empty a lithium battery, but not a SLA.
in that case the BMS will need to support the spikes, so a 60A should be on the safe side.

First, it is an electric mower not an electric start. No gasoline involved. The Mfg told me the motor will draw 'no more than 5 amps' The C rate of headway cells in 25. Room is limited on the mower so a 2p8s battery would be too big by a factor of 2. I would imagine there would be an amp spike if I get into some heavy grass and the motor starts to bog down but I think the 25C should handle it. I have finished the Batt and this weekend the weather looks to be in the 50s around here so I plan to hook it put and see how long it will run. No grass to cut yet and no snow to blow... for now.

The charger seems too low a voltage to me. And you need a charge controller to regulate the charge. If you directly connect the lead acid charger to the lithium cells, most likely the trickle wouldn’t be correct for the lithium.
What brand mower?

I was thinking the same thing about the charger. I could get a dc/dc constant volt amp board and run it through that or just take it to my bench top power supply. I can crank it up to 28v or more. The new LiFe power pack does weigh all that much to taking it to the bench is no big deal. The BMS is suppose to cut off the voltage when charged. The mower is a 2012 Wrox. The I have the SLA Battery pack that doesn't quit fit right but works OK and now a LiFe pack. This way I will aways have a freshly charged battery. My yard isn't all that big. Only takes me 30 min. to do, all walk behind. I only have to do it once per week and by July or August every other week and sometimes not the backyard at all because of all the trees and shade.
The real reason I am doing this is for my sailboat. Right now I have 2 12V AGM batteries that are OK for now but I like the idea of one being a lead starter Batt and having a Lithium house battery. I have a diesel aux motor with an Alt. 34 years old with only about 500 hours on the engine. New fangled Balmar Alt would be prohibitively expensive just to charge both batteries. The boat is usually at the dock with shorepower and a smart charger. I am thinking of a dc/dc charger like Will suggests if I go lithium house. I could then run the Fridge while sailing. It is a AC/DC unit that I only run AC at this time. It will keep cold for a few hours but any long excursions will start to warm up. The house batt will run radio, lights etc. which aren't much of a power hog. The Autohelm does use up some power which I use a lot on long trips so I don't use the Autohelm and the Fridge at the same time.

Update on my 24v mower. I finished the pack of using 8 38120 headway cells in series. Pack started with 26.4V. Last weekend was not bad for Feb so I got the mower out of of the back of the garage and ran it for 45 minutes. That is about 50% longer than it takes to mow the entire lawn. Voltage dropped to 25V. Looks like I have a good battery for years to come. I obviously haven't had to mow any thick and tall grass yet but I think these cells will handle it. There is a video on YT of a guy using 4 of these in series to start his truck and they list a C rate of up to 25.

Cool.
My two project mowers are being readied for spring mowing... sharpening the blades, and getting a battery that I want to use.
I have a 24v one and a 36v one... none are as convenient as the ryobi 40v one I also have, but I wanna play.