Snake and Pig.
Clash
Opposite signs: six years apart, facing each other across the wheel.
A clash is two signs directly opposite on the wheel. Traditional texts treat it as movement rather than doom: something between the two is always being pushed, changed or relocated.
Clash pairs are common among people who stay together for decades. What the clash asks for is room, since each person needs to be able to go and come back.
Beyond the year sign
We compared 200 Snake and Pig birthday pairs born between 1950 and 2005. The year sign changed little across those readings. Even classical clashes covered nearly the same range as harmonies.
The deeper charts varied much more. One person's nature could support, restrain, or closely resemble the other's. The year sign sets a tone. The two exact birthdays decide the pair.
What this leaves out
The year animal is one of eight characters in a Chinese chart. The day pillar says more about a person than the year, and a full reading compares all of them. The year also turns at Start of Spring, around 4 February, not 1 January, so people born in January or early February usually belong to the previous year's sign.