The weaker (AUX?) of the two batteries determines the ESR ( in micro/milliohms) and using the CCA rating is a rough measure of ESR rated @7.5V from full charge @12.5V or a 5V drop.
e.g. a 10 mohm 12V battery with 12.5V no load and 11.5V with a load is expected to draw 1V/0.01Ohm= 100 Amp. THe same is true of a battery switch with a 1 V drop and the total ESR of the 2 batteries is 10mA. It is expected to switch 100A until the charges equalize.
A Battery is expected to be in the range of 10k Farads or more with low ESR, but there is also a secondary charge layer (C2*ESR2) which gives rise to the memory effects with a certain time constant T2 in normal operating range. It gets complicated, but this is your main storage with a long time constant and the low ESR is short term storage. THis is why if you fail to start a car and the voltge drops down momentarily, it resumes to the previously voltage due to the bulk larger C which has larger ESR. THis larger C2*ESR2 affects your Ah capacity but not your CCA rating although they are somewhat related.
But if you keep the batteries in their normal range of 11.5 to 12.5 when not being charged, the max Current surge should be determined by the weaker of the two batteries for CCA rating. THis may be your AUX battery.
Thus Isw/1V(drop) =CCA/5V(drop) or Isw= CCA/5V
So if CCA is 500A , the switch ought to be rated for 100A surge. You can make it cheap Ammeters rated for this which use heavy copper shunts and sensitive 50kOhm/V galvometers but often sold for 50~100$
NEVER Use a 10A DMM unless you current limit with a tungsten headlamp bulb which acts a a constant current PTC.
If anything use a 55W Headlamp in series to limit the charge current so you can use a smaller A Rated switch or 25A Relay.
simulate this circuit – Schematic created using CircuitLab
Here you can use a 5A switch