Identical twins start out as a single egg which splits. This is why Jake could take over Tom's Avatar. Since the Avatar was grown using Tom's DNA, and Jake transferred in to it in the end, it is really the twins reuniting.The loss of an identical twin sibling must be so devastating, psychologically. So, Tom lives on in the Avatar, and it may have made his loss easier on Jake. Perhaps it gave an added strength to Jake, knowing that Tom was still "close".