It's not illegal. It's private land, the landowner makes up the rules, and breaking them is a contract breach - no more, no less.
Notwithstanding that, even if it's still subject to the Road Traffic Act as "accessible to the general public" there's no Law against bad parking.
Most (all I've seen) car parks stipulate on the T&Cs info where the meters are that cars must be parked wholly in a space. Buying two tickets is irrelevant because you're preventing 1 vehicle from parking. If everyone parked across two spaces the car park would only be able to hold 50% of the capacity. The car park owner might get the same amount in parking fees, but half as much footfall which would be far more valuable to them.
Notwithstanding that, even if it's still subject to the Road Traffic Act as "accessible to the general public" there's no Law against bad parking.
Most (all I've seen) car parks stipulate on the T&Cs info where the meters are that cars must be parked wholly in a space. Buying two tickets is irrelevant because you're preventing 1 vehicle from parking. If everyone parked across two spaces the car park would only be able to hold 50% of the capacity. The car park owner might get the same amount in parking fees, but half as much footfall which would be far more valuable to them.