If someone proposed a change to the current system of the president of the United States, like electing three presidents and needing two to sign off on each bill, for example, would this be possible? Could this be done through the standard methods of amending the constitution, or is there something preventing this from being possible?
The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.
We can see that there are only two situations where an amendment was ever blocked, and one of them expired in 1808.
We can therefore examine your proposal purely in light of the other limitation. Does it change the representation in the Senate such that it is not equal by state? No, it does not. So it would be a valid amendment to the constitution if made in the normal process.