When there is two neutral wires in one hole on the neutral bar of an electrical panel several things may happen. They relate to safety and should be corrected.
One is that the wires may be from two different circuits and that the loading of these two neutral wires may fluctuate. This fluctuation causes the wires to slightly expand and contract and over time the connection gradually becomes loose. Once in an awhile, if the connection becomes loose enough you may hear a snapping or sizzling sound.
If the wires become loose enough, then arching may occur. Arching creates heat and an equipment failure or a fire may occur. Should wires become too loose, in a few rare circumstances one may even slip out of the slot, resulting in a total loss of the neutral.
Panel manufactures and the code
The majority of the panel manufactures recommend only one neutral per slot. City building inspectors and home inspectors will normally call out this condition and recommend correction.
Below photos; one showing extra space in the neutral bar and the other that is overloaded.
This condition is usually one of the easiest to fix of all electrical defects. Often the fix is to pull one of the two neutrals out of the hole that it is in and place it in an empty hole on the neutral bar; provided that one is vacant. Very often there is one of more empty slots.
Should there be no empty slots, then a second or larger neutral bar may be added to the panel, providing many empty slots. On rare occasions a small sub panel may be added to cure this condition.
Many homes have neutral bars that have too many neutral wires in one hole or slot. Generally this does not create a problem for the homeowner, however, it is an easy fix and most home inspectors recommend correcting the issue.