If you’re referring to the description on the plan comparison page, it only says that sharing features included with a Premium plan include “Free Organizations [with] 2 users [and] 2 collections”. This table clarifies what is and what is not included in a “Free Organization” — if you scroll down to the “Integrated authetnicator (TOTP)” feature in that table, it should be clear that this is a Premium feature, which is only available to users who have an individual Premium subscription. Your Free organization does include 2 members, but your indiivdual Premium benefits do not transfer over to the other member of the organization.
Actually, you are lucky in that your total expense will only be the cost of two Premium subscriptions. Until about a year ago, if you wanted to use the integrated authenticator to generate TOTP codes for shared logins, this required a paid organization plan (even if both members of a Free organization had individual Premium subscriptions, neither member would be able to generate TOTP codes for shared items stored in the organization vault). Thus, until the change that was introduced last summer, your minimum expense for unlocking the TOTP functionality for shared logins would have been $40/year (Family plan) instead of $20/year (two individual Premium plans).