Police say the second device was found in a Mercedes in a Park Lane car park after it had been towed away from Cockspur Street, near Trafalgar Square.
Both devices were "viable" and "clearly linked", police said.
At a news conference on Friday evening, Deputy Assistant Commissioner Peter Clarke, head of Scotland Yard's counter-terrorism command, said the discovery of the second device was "obviously troubling".
Allah's tracking updates and, as usual, has the clearest, most comprehensive analysis you're gonna find on this in one place. He's showing off to earn an iPhone so go PayPal him or something.
A jihadi site was bragging about the bombing plot yesterday, but there's no definite connection documented yet:
CBS News found the posting, which went on for nearly 300 words, on the "al Hesbah" chat room. It was left by a person who goes by the name abu Osama al-Hazeen, who appears regularly on the forum. The comment was posted on the forum, according to time stamp, at 08:09 a.m. British time on June 28 -- about 17 hours before the bomb was found early on June 29.
Al Hesbah is frequently used by international Sunni militant groups, including al Qaeda and the Taliban, to post propaganda videos and messages in their fight against the West.