If a couple is married, but now wants to ensure each keep their
respective properties, is this possible? Can a “post-nuptial”
agreement be made that ensures property is kept by the original owners
This is often called a "marital agreement" although it is sometimes called a "post-nuptial." In Colorado and many other states, it is governed by the "Uniform Premarital and Marital Agreements Act." Colorado Revised Statutes § 14-2-301, et seq.
As defined in that Uniform Act (a National Council of Uniform State Laws model for states to adopt if they don't want to draft their own act):
"Marital agreement" means an agreement between spouses who intend to
remain married which affirms, modifies, or waives a marital right or
obligation during the marriage or at legal separation, marital
dissolution, death of one of the spouses, or the occurrence or
nonoccurrence of any other event. The term includes an amendment,
signed after the spouses marry, of a premarital agreement or marital
Colo. Rev. Stat. § 14-2-302(2).
The general rule is that:
A premarital agreement or marital agreement must be in a record and
signed by both parties. The agreement is enforceable without
Colo. Rev. Stat. § 14-2-306. Oral pre-marital and post-marital agreements, and unsigned pre-marital and post-marital agreements are not enforceable. Colo. Rev. Stat. § 14-2-309(8).
But, generally speaking, the person seeking to enforce it must also show that it was signed with full financial disclosure and disclosure of the risks involved in signing it, when each party has access to independent legal counsel, while not under duress, and that is was not unconscionable when signed, or when it was about to be enforced. Colo. Rev. Stat. § 14-2-309.
Colorado's formalities requirements are somewhat more strict the the majority rule of U.S. law, but in general, marital agreements are allowed in every state, but are highly restricted in how they can vary rights relating to children.
A waiver of inheritance rights at death that does not change the rights of the parties upon divorce, or a reclassification of specific community property of a couple from a community property state as separate property, can also be entered into, and is arguably a marital agreement, but is often done with fewer formalities.