Something I just wondered...
 

Why do the creatures on Pandora have 2 tendrils for Tsaheylu? Does it allow for two people to control the creature at once for some reason? Is it because the creatures use them as a sensory thing? I just can't work this one out... :S

I think it may be a backup or redundant. If you notice all the linking is done with the left tendril or queue. Just a thought, but it may be that if the left one is hurt, cut off etc... that the right one will mature and take it's place. That puts the Na'vi at a disadvantage with only one queue to work with.
Just my 2 cents.

I wondered that myself. I figure the animal life of Pandora, believe it or not, have a deeper connection with the planet; with Eywa, which is why (according to a theory) they came to the rescue when a neuro-signal was sent throughout the area that they were in danger.

I also figure (like you just mentioned) they use them to better use than the Na'vi do. While the Na'vi may use their queues to connect with Eywa on a spiritual level with the trees, with animals for taming, and with each other for mating, the animals use them purely for biological purposes. In Avatar: A Confidential Report..., direhorses are described as queuing up with each other to form herds, as do other pack animals of Pandora, some even using both queues at once with two other animals of the same kind to communicate. They may use them to sense changes in the environment (if there's danger near, for instance), or for linking with the trees for their own peaceful comfort in rest.

I really don't know for sure. The ikran can only bond once with a hunter, which begs the question, "Why two queues if only one hunter?" That's another thing. Perhaps the double-queue on each animal is a practical survival tool, where if one is severed, they still have another to use (like Di' Ana mentioned). The Na'vi don't need two because they're more mentally agile and adept. The ikran might use their queues for other purposes rather than the Na'vi; perhaps the examples I gave are also reasons for the ikran double-queue.

But honestly, I can't be too sure. We'll have to wait for Cameron's prequel book to really know anything.

I'd guess, it's just evolutionary. Having more than one would make sense from a survival perspective. Also, they are prehensile to some degree (such as the direhorse using one to get at the flower it was drinking from), so as they have a useful function, evolution of symmetrical ones is likely to occur

It makes sense for them to have two, especially since they queue up together in herds (some of the animals). It also seems to be used to as an extra limb. I think it also helps symmetrically too.

that and what di'ana said. i think from evolution form it is more usefull to have two ones... to grab things when there are still two of them, and to have another one when the other one is lost for the use of reproduction/getting to do things that creature does. i can imagine that it is hard with one tendril, but smart animals like direhorse can learn themself to get to their food with one tendril i suppose.
and for the artsy thing, i think it looks better when there is symmetrie, though my friends from my artgroup all agree(without exception) that uneven numbers make a visual more interesting to look at. so that might also be the case. two makes it easier to look at because most people seek symmetrie.
for na\vi.. they seem to be more evolved than the creatures that have two tendrils, so maybe they have evolved to have only one queue, but who can tell us that the braid is not covering two queues. they might have grown to be one through the ages....

For lefties...