The Shelley 4PM won’t handle the amps required and there is absolutely no volt, amp, or watt specifications at that Aussie website you linked to, that gives any clues on capacities and capabilities - a distinct way to discourage sales! No links to datasheets or specifications!
Aussie three phase is 415volts, so devices rated at 400 volts may be sailing a bit close to the wind. Jump to the next higher common voltage of 600 volts will allow a margin of comfort.
From the questions you are raising and the possible solutions you are offering, you appear to be a little out of your depth, and could I venture it will be far cheaper to consult an experienced licenced electrician who (by Australian law) MUST be engaged to do the electrical works you need anyway. They will understand that the contactor needs to be derated for suitability in your climate location, and which contactor to buy to switch the voltage and current you need and wire it safely to Australian Standards.
Colloquially: You are shopping at Bunnings, but you should be shopping at Middys. You are asking for advice at a website where people are offering solurions that might work for a thousand or so watts, but you are needing to switch many upwards of FIFTY thousand watts. If you get it wrong, you only have two expensive cars, and maybe your house to offer to the fire gods, and insurance will be asking many questions.
When in doubt, ask a professional. You may find an industrial contactor (I’m guessing a 600volt, 100amps, three pole contactor with 240 volt input that is driven by the 240volt relay controlled by HomeAssistant might do the job) will be ten percent the cost of a fancy Shelley that grossly underperforms for your requirements. Tie that in with a cheap Tuya style 240 volt single pole switch (or cheap Shelley if you must) to switch the contactor, and put it out of the sun and rain, along with robust wiring. Your misunderstanding of the number of channels (1) and contactors (only 1 device with multiple poles) required also confirms this.
You haven’t said how these devices are going to be hooked together. Do you have manual switches, different connectors you unplug, and how do you exclude the possibility of both cars pulling current at one time from the mains or battery? Do you now expand the number of contactors and channels, or have some sort of interlocking controller that accepts commands from HomeAssistant but follows its own safety rules in case your power is interrupted and comes back with catastrophic ‘all devices on’ status? How about a few milliseconds between switching one contactor off and another on? You wouldnt want a pair of contactors to short that much current - even the MIG welder on special at your local Aldi supermarket only uses two thousand watts, not fifty thousand. Think of how many sparks it could ignite.