As far as I know only citizens of former Spanish colonies are allowed double citizenship with Spain. However, in real life how likely it is that if you take the Spanish citizenship, and you have not formally renounced your U.S. citizenship before a U.S. authority, that your U.S. passport will be taken away from you. Actually, I think the U.S. itself doesn't allow you to have another citizenship. However, other countries do. For example Mexico, the many sons and daughter of Mexicans born here in the U.S., are allowed, but under Mexican Law to have both citizenships. However, if you are living in Europe, commit a crime under U.S. law and the U.S. tries to prosecute you under its (super) long arm statute and bring you back home, and you claim you have renounced your U.S. citizenship to become Spaniard, that's something different, and more likely than not your U.S. passport will be taken away from you, and you will be prosecuted anyway because an exemption to the rule is that you quit your citizenship to evade the law (like paying your taxes).
There are many U.S. citizens who also are English, Mexican or Canadian. I might be wrong, but think that the only real life situation where you will endanger your U.S. citizenship is when you claim before a U.S. authority that you are a citizen of another country. Hope it helps….For specific legal advice talk to an attorney!