They largely could. My wife is a lawyer at one of the largest law firms in the world, and a lot of what the trainees are doing could've been done by a PA or secretary, and much of it is certainly trivial to partition.
I'm sure there are legitimate reasons for some people to work long hours sometimes, but especially for the larger firms and at the lower pay grades, it's mostly a question of what makes the firm more profitable.
The reason they don't put more people on it unless they absolutely have to is money. They bill out people by the hour, but pay them a flat rate unless they are equity partners in the firm.
The harder they work them, the more they bring in for pretty much the same cost (except free dinner and paid taxi home after certain hours...), and at the same time, there's rarely an incentive for them to get their staff to work fast, as long as quality is good enough - if they're exhausted and slow down and spend more hours, it's more money for the firm.
While trainee salaries at the firm she's at isn't that far off what was mentioned above, due to the number of hours, trainees there are in effect making less per hour than the secretaries, and it takes several years post qualification to get to a decent hourly rate.
The reason they stay? A combination of wanting to make partner, which in this firm will make you millions a year, and a culture that frames it as personal failure if you don't meet deadlines that makes it impossible to actually stick to your contracted hours.
EDIT: So to specifically your question, of why not two people at half the price. People going for these types of jobs are at least in part driven to go for the high salaries. Since the pay per hour is laughable for the amount of education, if they were to halve salaries and hire twice as many people, they probably would not have many applicants.