The second vessel is the Norwegian-owned Bow Asir and far larger at 23,000-tonne.
It is reported to have 28 people onboard including a Russian captain. It is not yet known what cargo is being transported.
The ship was taken by a gang of 20 pirates carrying machine guns, reports said.
More than 20 ships from the EU, Nato, China and Russia now patrol the waters off Somalia and around the Horn of Africa, a busy shipping lane plagued by pirates.
Shipping routes off the coasts of Eastern and Western Africa have become increasingly dangerous, with the Gulf of Aden, Gulf of Guinea, and the coasts of Cameroon and Nigeria becoming the most risky.
Dozens of ships seized by pirates have been released after owners paid tens of millions of dollars in ransom.