The show, the first in Five's new documentary series, followed 57 year old conjoined twins Donnie and Ronnie as they embarked on the trip of a lifetime.
It was beaten by the last in the series of BBC One's Nature's Great Events, which took 4.02m (18%), making it the most-watched programme of the hour.
Grand Designs continued to perform well for Channel 4, pulling in 3.89m (17.4%), while ITV1's Taggart drew 3.63m (16.2%).
The final instalment of BBC Two's Building The Olympic Dream was last for the hour, pulling in a lowly 760k (3.4%).
The latest episode of The Bill picked up 4.82m (20.8%) for ITV1 in the 8pm hour, and on BBC One, Waterloo Road interested 4.43m (19.1%). Relocation, Relocation had 3.19m (13.8%) for Channel 4, and BBC Two's Orangutan Diary drew 1.94m (8.4%).
Five was last for the hour with 1.4m (6%) for Ice Road Truckers.
Overall, ITV1 led primetime with an average share of 21.9% to BBC One's 19.4%. Channel 4 was third with 11.7% (+1: 0.8%), followed by BBC Two with 6% and Five with 5.7%.
Elsewhere, the new season of The Unit grabbed 272k (1.8%) for Virgin 1 at 10pm, and 236k (1.2%) tuned in to ITV2 at 9pm to watch the latest Gossip Girl episode.
Ratings data supplied by Attentional