No, removing the earrings does NOT nullify the fusion.
Source - http://universaldragonball.wikia.com/wiki/Potara_Earrings
Quote from the wiki -
The earrings have the ability to fuse two people into a single entity, substantially enhancing the abilities of both individuals. The resulting fusion lasts forever, even if the fused being removes the earrings. However, there is one instance where the Potara Fusion will be nullified and restore the fused being back to their original forms, which is when they are sucked into an environment that would not support it (as seen with Vegito being absorbed inside Buu).
Edit: I've found a quote of Elder Kaioshin, stating that the Potara earrings are meant to be eternal-lasting.
Context: Elder Kaioshin gives the Potara to Goku Elder Kaioshin: “Here! Put
this Potara on your left ear! [ ] Put the other one on Gohan’s ear.
Just by doing that, you two will be able to merge together! Like with
Fusion.” Goku: “Huh! Re-really!?” Elder Kaioshin: “Of course. And
what’s more, the effect is greater than with Fusion! This has been the
trump card treasure of the Kaioshins since long ago.”
source - Chapter: 501 (DBZ 307), P6.2-6
Context: Goku asks how long Potara-based fusion lasts
Elder Kaioshin: "The Potara don't have such a weakness. It's eternal! You'll never return to normal again!"
source - Chapter: 501 (DBZ 307), P11.6
Edit 2 -
As jphager2 has mentioned in the comments below, after using the Potara Earrings, they can then be removed & re-used to fuse 2 new people, we've never been given information about the possibility of 2 fusions being able to fuse, and the only information I can find on it is speculation.