Can a person buy 2 term plans from 2 different companies? Like one from HDFC life and one from LIC ?
As far as the total sum assured is within the total sum assured you're eligible for, you can technically buy 2 plans.
Having said that, there is no major advantage in trying to diversify your policies.
In fact, it'll mean you'll go through the whole process of buying the policy, customising and documentation twice - plus, your family will go through two separate claims processes as well. If possible, it is best to take one good policy.
I learnt this from this eBook on term insurance - The Ridiculously Simple Guide To Term Insurance (beshak.org) Can read in detail about it, if you'd like. :)
How total sum assured is calculated one is eligible for ? Is it something like 25 times of one's yearly earnings?
Various Insurers usually use a multiple like 20X, 25X, 30X of your annual income, based on your age, occupation.
If you want to know how much term insurance you should buy - you should check this article: The most effective way to calculate your Term Insurance Cover (beshak.org)