Firstly, there are two species of haggis. One with the left leg longer and the other with the right leg longer.
A pair of haggis legs, like a brace of pheasants, wouuld consist of one of each. Thus parity in weights.
Obviously, they cannnot interbreed - the balancing act required for procreation would be nigh on impossible - like standing up in a hammock.
Each species has to go round the hillside in a different direction and eggs are laid in specially scratched out furrows.
In very high winds and occasional earth tremors some eggs have been displaced and may, if you are very lucky, roll down the hillside into a hedgerow.
Good luck with your search.