Mikvaot, Chapter Six, Mishnah Nine
1) If a wall between two mikvaot had a perpendicular crack, [their waters] may be reckoned together;
a) [If it was cracked] horizontally, they cannot be reckoned together, unless there is at one place [a hole as big] as the tube of a water-skin.
b) Rabbi Judah says: the rule is reversed.
2) If there is a breach from one [mikveh] to the other, [they can be reckoned together] if the height is as [the thickness of] the skin of garlic and the breadth like the tube of a water-skin.
Section one: According to the first opinion, perpendicular cracks between the walls that runs add up together, such that if all of the cracks add up to the size of the tube of a water-skin, then the two mikvaot can be reckoned together to reach the requisite 40 seahs.
However, if the crack runs horizontally, then there must be at least the size of the tube of a water-skin at one single point for the two mikvaot to be joined.
Rabbi Judah says the reversehorizontal cracks do join together to add up the requisite size of a whole, but perpendicular cracks do not join together.
Section two: For a breach to serve to connect two mikvaot that have a wall separating them, the breach must have a very minimum height, only as high as the skin of a piece of garlic. However, it must be as wide as the tube of a water-skin.