When Goku, Gohan, Vegeta and Trunks prepare to train to face Cell Goku warns them that the Hyperbolic Time Chamber is dangerous enough that they can't train alone in it.
But there's no life in there. just a building and open empty white space. how exacting is it too dangerous to train in there alone?
My answer is only opinion based but i think it has to do with the fact that nothing is there besides white emptiness. The "danger" comes from the psychological nature, as a solo person would eventually go crazy without any object reference. Everywhere around you is just an endless whiteness so that most likely takes a huge effect to your mental state.
Anyway that is the only reason that comes to my mind.
The gravity inside the Time chamber is more than your average Earth. What if someone fainted in the between of no-where? since it's vast.
It would be safer for two people to train at a time.
The conditions within the room's training area are extremely punishing on the body. 10 times Earth's gravity, temperatures that fluctuates rapidly between -40 and 50 degrees Celcius, and 1/4th the air pressure of Earth; try doing an intense workout while during arctic, desert, and high altitude conditions at the same time with extreme gravity added on top of it. Then you add the psychological toll of being in such an empty environment for extended periods.
The Hyperbolic Time Chamber has only one door for entrance and exit. If someone trains alone and go too far from the entrance, he will be locked there forever.
