Depends how big the trees are and how big you want them to get. One could end up growing more vigorously than the other, so you could end up with a lopsided tree. I say tree because as they grow it will look like one big tree. Is there going to be cement or something at the bottom of the tire to prevent roots from growing into the ground? Since it floods I would mound the soil up, then place the tire on top. Or you could plant them somewhere where it doesn't flood. It's your call. You could get some cinder blocks and place them on there sides in a square fitting under the tire so you can't see them. Place the tire on top and fill with a fast draining soil. Then fill in the sides. Or you could build a raised garden bed, then you could plant a "forest" of plumerias. Just use cinder blocks and cement to keep them in place. You could find many tutorials for step by step instructions on how to do this on youtube. Hope this helps. Good luck :)

The cinder blocks are a great idea that would give me extra height as I plan to raise the ground over time. The tires im using are about 5' in diameter and 21" wide. I figured to let the roots grow down through. I would sure love to see some pics of multiple plumerias growing together.

Wish I had some. Most of my plants are 5 feet and under. Not trees yet. Really these plants turn into bushes unless you live in like Central America or Hawaii. Then they turn into trees. I think a raised planter bed is you best bet...

Out on the coast here there are true plumeria trees more than 30' tall. Im about 6 miles inland and I see many plumeria 8-10 feet tall. I have planted a bamboo northern wind break and plan to plant my plumerias 10' south or this for frost protection. I am hoping to create a microclimate so I can have healthy 10-15' small trees. My bamboo will be 35'-40' tall in a year or so.

Sounds like you have a good plan. Please update us on how they get along!