It's an analogy, it makes no difference. The charging rate is double with two separated battery packs regardless of the capacity of the batteries.
Put two half sized isolated packs in one car with two charging ports connected to two chargers. They charge twice as fast, inherently.
Only if the capability of the chargers is the limiting factor...
Look at the graph shown above, relative to the charging rate of a Model 3 Long Range plotted against the current charge state of the batteries.
A Tesla v3 Supercharger can go 250kW. The batteries, for a VERY short time and portion of their charge capability, can exceed 250kW, but from about 21% charge until 100%, the battery is the limiting factor, not the charger.
As mentioned, you can't look at a Tesla Model 3 LR as one "battery pack". That battery pack consists of hundreds of battery cells segmented into multiple cells per module, and the modules are then built into the overall "battery pack". I believe they're isolated at the module level. But the key factor is that the cells themselves CANNOT take charge beyond a certain rate.
From here: https://evannetwitter.com/blogs/news/understanding-teslas-lithium-ion-batteries
One of the key requirements for electric car batteries, especially on road trips, is that they need to be recharged relatively quickly. Since batteries are direct current (DC) devices and home electrical service is AC, charging at home typically uses a 240 volt circuit supplying 40 amperes (about 10 kW of power). The car has built in charging circuitry that rectifies the AC, converting it to DC. Charging this way typically takes several hours. Tesla has installed Supercharger DC charging stations worldwide that supply up to about 135 kW of power. The DC bypasses the car's charging circuitry and charges the battery pack directly. This is much faster, requiring 20 to 40 minutes typically.
The Tesla battery packs using Panasonic 18650 batteries can charge no faster than this. The maximum charging voltage for a Panasonic cell is 4.2 volts. Panasonic specifies a maximum charging current of 2 amperes per cell. Tesla allows charging current to be up to 4 amperes. Therefore the maximum power that a Tesla battery pack can can use for charging is 4.2 X N X I where N is the number of cells in the pack and I is the maximum current allowed per cell. For 85/90 kWh packs this is 7,104 X 16.8=119.3 kW. For the 100 kWh packs it is 8,256 X 16.8=138.7 kW. There is no way to charge faster without increasing the maximum charging current per cell which might hasten degradation of the cells or worse.
The Tesla Model 3 cars will use the Gigafactory manufactured 2170 cells mentioned above. The larger cells may be able to use more than 4 amperes of charging current which would hasten charging but, because the 2170 cells have more energy storage capacity than the 18650 cells, proportionately fewer will be needed to create a pack with a given kWh rating. (N gets smaller, I gets larger). This means that higher power charging is meaningless for these battery packs. The 4.2 X N X I relationship still applies. It will be interesting to see how these new battery cells perform.
Note that I was off by an order of magnitude or so... Tesla battery
modules consist of hundreds of cells each, and Tesla battery
packs consist of thousands...
And that 2 A/cell or 4 A/cell is a maximum rate. As the above graph shows, charging slows down as you fill the battery, so this is merely PEAK charging rate.
If you split the same battery pack into two battery packs without adding cells, you have not added aggregate charge rate. Therefore isolation doesn't allow them to charge faster.
