Your Daly contact is definitely wrong, with respect to two fully independent batteries in parallel at loads which do not exceed the BMS limits of a single battery. I own and use such a configuration myself. One battery is 106Ah with 100A Daly BMS, while the other is 123Ah - also with 100A Daly BMS. They work fine together.
Perhaps he/she was thinking of possible 'load imbalance' and unbalanced charging, with one of the small batteries being favored over the other. That will be present, to a small degree, due to differences in the internal resistance of the 4-cell "string" within each battery, including the bus bars, fuse, BMS, and internal terminal wiring of each battery. External wire lengths (and resistance) can be balanced by you.
If you're super finicky and can measure resistance VERY accurately, you might even be able to "make up" for battery-internal differences by slightly un-balancing your external lead lengths. But I never bothered to do that with my own pair of not-quite-the-same 4S batteries.
In a slightly unbalanced configuration, such as mine, one is certainly being "favored" over the other during discharge, and also during charging. But when they become nearly full-charged (exceeding 3.4V per cell and invoking BMS cell balancing) the "lower battery" will catch up, because the SOC affects charging voltage a lot (near the end of charging). Likewise, at the low end of SOC the less-used battery will be providing higher Voltage (and higher current) into the shared loads. It will be drawn down faster, and they will both reach 15% SOC (or less) at roughly the same time.
Building 2 smaller batteries does create some "redundancy", if you are paying close attention to actually see that one battery "failed" or misbehaving (while the other is still working). But, although the single BMS costs more, I think that the total costs of 2 smaller batteries are roughly the same. In my own case, I needed to buy two heater pads for low temperatures, 8 sets of compression bolts and springs, two fuses, and I had to build two cases. A single large battery pack would need only one heater pad, 6 compression bolts and springs, and one case.
I also have a 3rd battery planned, and awaiting bigger cells from Michael. Excluding the cells costs, it's build costs will be LESS than the sum of costs in my two smaller batteries. You could fit "2 batteries" in one case (with 2 sets of terminals), but that would be kind weird.
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Bottom line: Building a single 4S2P battery, as they suggest, does reduce redundancy (while increasing current capacity). But within the battery case, the individual pairs of cells have the same variability of internal resistance against each other - and one may be favored over the other in actual usage. That's approximately THE SAME issue, on a cell-by-cell basis, which they may have in mind when warning that two separate 4-cell battery packs can be used in an unbalance way. The one big advantage of their scheme is the ability to use all "300A". In a dual configuration with a lot of current load, the draw on one battery will definitely reach and exceed "150A continuous" (or the corresponding short-term limit) and cause BMS shutdown before the other one has reached 150A of current load. The situation will shuts down battery #1, and then the continued load shuts down battery #2 very rapidly. The build costs are comparable.
The advantage of your scheme is "redundancy", if you won't be needing much more than 150A most of the time. Its disadvantage is possibly provoking BMS shutdowns, one after the other, when you try to exceed the individual current limits. Will you be using much more than 150A? The more you depend on "load balancing" among the two batteries at high current draw, the more you should prefer to build it as a single unit with the bigger BMS.
