Binary vascular system
A binary vascular system was a type of circulatory system in which all the blood vessels lead to two hearts. Time Lords were perhaps the most notable species known to possess them. (DW: Spearhead from Space, Robot, The Christmas Invasion)
The Doctor once encountered another double-hearted near-Human species called the Dulcians. (DW: The Dominators) The Kirithons were genetically engineered by the Panjistri to have two hearts. (NA: Timewyrm: Apocalypse)
In general, binary vascular systems were not common amongst near-Humans. In the Mutter's Spiral galaxy, a few monopod species and a drone species also had two hearts. (NA: Tragedy Day)
The full implications of a binary vascular system were unclear. However, the Dulcian Rago once told Jamie McCrimmon that beings with single hearts were physically weaker than those with two hearts. (DW: The Dominators)
Similarly, when Henry van Statten performed an invasive scan of the Ninth Doctor, he marveled at the design and enthusiastically announced he was going to patent it. Given van Statten's obsession with superior technology, this suggested that two hearts were, literally, better than one. (DW: Dalek)
Immediately after his regeneration, the Eighth Doctor was able to tear a heavy metal door from its hinges, perhaps indicating that Time Lords' vascular systems provided greater strength than a human's. (DW: Doctor Who)
An adapted form of CPR, in which both hearts were included in the life-giving rhythm, was once effective in restarting the Tenth Doctor's hearts. (DW: Smith and Jones) On at least two occasions, the Doctor survived, albeit in a greatly weakened state, while one of his hearts failed. (DW: The Christmas Invasion, The Shakespeare Code)
A binary vascular system provided some level of protection from attack, in that single-hearted near-Humans failed to aim correctly when attempting a close-range attack. The Fifth Doctor, for instance, was once stabbed in the chest, but survived because his Human attacker aimed for the spot where the Human heart would be. Because the Doctor's two hearts were in a slightly different location, the attack was ineffectual. (BFA: Son of the Dragon)
