Mikvaot, Chapter Seven, Mishnah Six
1) If a mikveh contained forty seahs exactly and two persons went down and immersed themselves one after the other, the first becomes clean but the second remains unclean.
a) Rabbi Judah says: if the feet of the first were still touching the water, the second also becomes clean.
2) If one immersed a thick cloak and when he drew it out a part was still in contact with the water [and then another person immersed himself in the mikveh], he becomes clean.
3) A pillow case or a cushion of leather, as soon as it is taken out of the water by its open ends the water which still remains in it is drawn water.
a) How should one do it? One should immerse them and draw them up by their lower edges.
Section one: When the first person exits the mikveh, he will inevitably remove some water with him. The mikveh will now have less than forty seahs of water, so the second person to immerse will not become pure.
Rabbi Judah says that if the feet of the first person are still in the mikveh, we consider all of the water on his body to still be in the mikveh. In such a case, the second person to immerse is pure.
Section two: This matches Rabbi Judah’s opinion from above. As long as part of the thick cloak is still in the water, the water that is on or soaked up by the cloak counts as if it was still part of the mikveh.
Section three: The pillow case and leather cushion both have a receptacle. Therefore, as soon as they are immersed in the mikveh and some of the open ends are removed, the water in them is considered drawn water. This would disqualify the mikveh if it wasn’t large enough, meaning if there wasn’t forty seahs of valid water.
The way to fix this problem is to immerse the pillow case or leather cushion with the open side down, such that when he pulls them out, there is no water inside. Simple problem, simple solution!