Lots of people grow DF in containers. Check out the Red Dragon Fruit Co. If your fence post has some height to it, you should be okay. Not sure what you meant when you said "to be more like one plant". Treat each variety as a separate plant. They may grow at different rates. Prune when they reach the top. You may want to attach some sort of support onto the top of the post to allow the branches to spread out more. Good luck.

I'm not familiar with growing Dragon Fruit, even though I'm growing two plants in hanging pots, but if they grow like epiphyllium, which they resemble, they need to have roots that fill a container and become root bound before they will bloom. If they are grown in the ground, this won't happen. The link below shows them growing in pots in Thailand.

Here is a link that might be useful: Growing Dragon Fruit in Pots