So yeah if only half current is used for two then surely they can both connect to a junction box and only one cable needs to go from that to only one RCD . It makes no sense to me that two parallel wires with two separate RCDs are needed because that one wire will only be carrying the same maximum current. Unless each of them can do a random spike I guess .
This is a complicated subject, but you need to be thinking about overcurrent protection and RCD protection separately.
Certainly you can have a single 32A-rated cable from the house to the garage and split it there for the two chargepoints; the issue is what switchgear you have to install at each location and what type of cable you have to use.
Tesla quotes IEC60364, but the UK wiring regulations are based on that nowadays and the same requirement appears, requiring you to have a separate 30mA RCD for each chargepoint. The need for no higher than 30mA RCD is for safety in the event that you are able to touch a damaged cable or similar: 30mA is the minimum sensitivity considered sufficient to save the life of the typical person in that scenario. The specification of two separate RCDs rather than one shared one is not a safety issue; I can't be certain what was in the mind of the people writing the regulation, but there are two things they might have been concerned about: selectivity - not wanting a fault on one EV to knock out all charging at that location; and concern that the natural leakage from several non-faulty EVs might add up to more than 15mA (noting that an RCD marked "30mA" is actually "guaranteed to trip at 30mA, guaranteed not to trip at 15mA, might or might not trip at levels in between). All EVs will have some leakage in their chargers; the Renault Zoe is notable for having a particularly high leakage due to the unconventional charger design. Personally I think the 'RCD for every chargepoint' is overkill for a domestic 2-chargepoint situation, though very sensible for larger commercial sites, however that's what the regulation says (though since its not a safety issue you could in principle go with a documented non-compliance with the regulation since the only legal requirement is to be safe).
So, sticking with the regulations you could select a suitable cable to run from the house to the garage that didn't itself need RCD protection (armoured cable, for example), have _no_ RCD at the house end (only overcurrent protection, ideally a fuse), then at the point where the circuit is split for the two chargepoints (in the garage) you have a small consumer unit with two RCDs. The regulation also requires separate overcurrent protection, so either a pair of MCBs as well or else use RCBOs in place of RCDs. The separate overcurrent protection is almost pointless in this scenario as it can't possibly be selective with respect to the upstream device using breakers, and is a pretty tall order even using a fuse upstream and a breaker downstream, however these devices are cheap.
Note also that with the current generation of Tesla WC the two RCDs are going to be rather expensive, as since 2019 those RCDs need to be Type-B unless the chargepoint has internal DC leakage detection (which most other manufacturers have been adding to their chargepoints recently but Tesla doesn't yet appear to have got around to).
