A team has finite resources, everything has an opportunity cost.
If the team were to run two cars, they would have to build two of everything, which means they won’t be able to produce as many variations of parts.
Considering that most parts at this stage are prototypes, building two of something that might not work is a waste of resources (time, money and staff).
The benefit of running two cars at a test is negated by variables such as setup and driver, so any analysis between the cars would have to factor in that, making it less accurate.
@Michael Since they aren’t competing for the best lap time at testing, the car’s setup is handled differently. They aren’t looking for the fastest way of getting around Jerez, they are looking at a whole bunch of measurements with a variety of setups. Not having to find the optimum setup for that track allows them to run setups for anything from Monaco to Monza. They’re discovering how the car behaves with setup changes, which helps them dial a car into the track at races.