CPUs in the automotive space are primarily chosen for being cheap, and with the right number/kind of IO. If you do enough stuff¹, it's certainly possible to reach the CPU load limits of a single core CPU, and it might cheaper to upgrade to a dual core than put in another ECU (also, faster ECUs with the same IO/arch might not be available).

I've also seen arrangements where the dual-cores have separate memory spaces, and one core acts like a safety processor/monitor while the other is the main compute module.

1: examples of CPU intensive stuff include active damping, active aero (spoilers), active rear steering, steer by wire, etc. You may think these things are irrelevant w.r.t. an engine ECU, but if the engine ECU is responsible for producing torque, then systems like traction control, stability control and etc are all directly related to torque production