It sounds to me like they are showing behaviour consistent with breeding.
1. They have "taken" over the coconut shell
2. Vibrating next to each other - the female usually does this to entice the male
If both of them have coloured up (showing more colour) this is also a good sign of them breeding as colours normally become more vibrant during mating.
If they are aggressively guarding the coconut shell (normally if there are other fish in the tank they will show this behaviour) I would say you might even have some eggs laid.
Keep on eye on your shell....Eggs normally hatch in about 4-5days dependant on temp etc so you may have some fry swimming about shortly.
Do you have them in with other tank mates?
Hope this helps...