Try moving the stake away from the trunk about a foot or so, and tying it up with something elastic--heavy-duty plastic tie, old bicycle inner tube, etc.--so the trunk can sway a little. The repeated slight bending of the trunk releases growth hormones that help thicken it. A little extra potash in the feeding can also speed up the process, but it can still take 2-4 years before the "splint" comes off! Good luck!

I have a 5 ft. double trunk jacaranda that I've tightly bound together in hopes that they would eventually become one. Does anyone think this will work or should I just remove one of the trunks. I know that a double trunk will eventually split due to the excessive weight.

The two trunks will never fuse together. Co-dominant leaders like this often have 'included' bark at the point where both trunks originated. This inclusion is a weak point that is doomed to failure. I would select the strongest trunk and remove the other as close to the point of origination as possible.

Here is a link that might be useful: Jacaranda

Thanks for the great responses! I think I should remove the weakest trunk especially since we live in an area that gets hit often with hurricanes. My mother-in-law and I are so excited to be the first in our town with Jacarandas. She brought the seeds back from California a couple of years ago and our trees have survived 2 hard freezes but that's only b/c we protect them like babies.