Day 5 – Berachos 6
The Gemara tells us that when 2 people are learning, their learning is recorded in the Sefer Zichronos. If one is learning alone, the learning is not recorded in the Sefer Zichronos.
Some Meforshim explain this to mean an actual record book in the heavens, and the need for two people is in order to sharpen each other to arrive at the truth, which can then be recorded.
The Meiri and Rav Tzadok quoting the Iszbitzer explains that when one learns with a friend, you are refining the learning until it is very clear to you. This learning then gets embedded into ones memory.
However, when one is learning alone, the learning does not necessarily get remembered as well.