Travel to Cuba as dual citizen (USA+other)?

The Cuba Information Manual ("The Definitive Guide to Legal and Illegal Travel to Cuba") says:

The embargo laws do not forbid U.S. citizens from traveling to Cuba. They do, however, forbid U.S. citizens from spending money there without the proper permits, which essentially amounts to the same thing—unless you plan on begging your way around the country (from Cubans who don’t have much themselves) and sleeping on cardboard in the parks. Even if you brought your own food, you’d still violate the law by paying the twenty-five dollars to the immigration guy for your tourist card. You even have to pay to get out: twenty-five bucks for a “departure tax.”

The penalty for violating the embargo law is stiff: Up to ten years jail time, $250,000 in criminal fines, and $55,000 in civil fines for each violation. So that café con leche you drank at the Hotel Inglaterra in Havana could end up costing you a bundle if a U.S. customs officer finds the receipt in your pocket. The U.S. restrictions on spending money in Cuba apply to all citizens and residents of the United States—no matter from what country you travel to Cuba and even if you have dual citizenship. The restrictions also apply to non-U.S. citizens physically in the U.S.

As an American citizen you are required to the follow the laws of the USA despite any other nationality that you may have. Most laws don't apply to citizens residing abroad, but some do. Perhaps the most significant is paying taxes on world-wide income, but also includes participating in the selective service (military draft), reporting foreign bank account balances and respecting embargoes on various countries. There also some recent laws which involve restrictions and punitive measures for US citizens engaging drug use, trading in pirated merchandise, or sexual activities with minors abroad, even when these practices would not violate laws in the country where the event happened.

Regarding Cuba, specifically the official regulations state:

They apply to all persons (individuals and entities) subject to U.S. jurisdiction –
including all U.S. citizens and permanent residents wherever located, all
persons in the United States, and all branches and subsidiaries of U.S.
organizations throughout the world – as well as all persons engaging in
transactions that involve property in or otherwise subject to the jurisdiction
of the United States.
    ...
Unless authorized by a general or specific license, any person subject to U.S.
jurisdiction who engages in any Cuba travel-related transaction violates the
Regulations and may be subject to penalties.

In reality if you are not living in the USA and have no substantial contact such as bank accounts, residence, etc. nothing will probably happen, but it is not strictly legal.

EDIT: This post goes into more details: Are there any US laws that a US citizen MUST obey while traveling, even if those laws do not exist in the country he is traveling to?

As an American citizen, you are bound by the laws and embargos of the U.S. no matter which passport you present. So, you would technically be breaking the law by visiting Cuba and spending money.

As per the Treasury Cuba Sanctions

Unless authorized by a general or specific license, any person subject to U.S. jurisdiction who engages in any Cuba travel-related transaction violates the Regulations and may be subject to penalties.

Presenting your non-us passport may make the process of visiting even more painless should you decide to do it illegally since you won't have odd stamps in your U.S. passport.

As an U.S. citizen the only way to enter in Cuba legally to the U.S eyes is by using the “people-to-people” program authorized by the OFAC. These trips are not touristic.

It’s then illegal for you as U.S. citizen to enter Cuba no matter which passport to use, however you may enter and U.S. government will never know, but that will be illegal:

If you fly to country X, withdraw some cash, and then buy plane tickets to Cuba; when entering Cuba you can present your U.S. passport or your X passport, it doesn’t matter, your passport will not be stamped unless you request to customs to stamp it. You spend only cash in Cuba. Then you come back to country X. When you go back to the U.S, they will never know you visited Cuba. But that’s illegal.

Any American citizen (even if you have dual citizenship you are still an American citizen) requires a license to travel to Cuba (http://www.treasury.gov/resource-center/sanctions/Programs/Documents/cuba.pdf).

There was a loop hole that allowed American's to travel to Cuba without a license while being "fully hosted", i.e. having someone not subject to US jurisdiction pay for everything related to your trip (http://www.treasury.gov/resource-center/sanctions/Documents/tab4.pdf), but this has since been removed (http://www.futurodecuba.org/CUBAN%20ASSETS%20CONTROL%20REGULATIONS.htm).