Q.jujube pruning and container planting
I purchased two jujube trees over the weekend. They both will need to be grown in containers. In retrospect, I am concerned that the two trees I purchased are already too leggy and don’t have many branches. I plan to replant them in separate 2x4x2′ horse troughs (with drainage holes). I can’t find a good resource on pruning them. The trees are about 6′ tall and the first branches don’t start until about 3′. When I plant them in a container, they will be 7.5′ tall. If I prune the top foot or so, will that encourage branch growth? And what is the recommended container size (depth, width), will a trough be suitable? Thank you!

Certified GKH Gardening Expert
Jujube are medium sized trees, up to 40 feet, so planting in a container is not the best growing conditions for long term. But if that's what you have to do then the troughs sound adequate to get them started for a couple of years.
Water will be your (and the tree's) biggest challenge in your desert location. In containers that could mean daily in the summer. Frequent watering will tend to leach nutrients, so use a good rich potting soil that retains soil moisture and doesn't drain too rapidly, and regular fertilization will be important.
The tall spindly growth is natural for a young tree and if left alone will naturally start to fill out the lower branching in time. I would not recommend pruning too heavily. Your idea of reducing by topping one foot is not all bad, but don't take any more than that or it will force a vigorous growth response and extreme bushiness at the top that will require more maintenance. The minor height reduction, even if only a few inches, will suppress apical dominance and allow for more lateral branching.
From the photo it appears that you are in an open, possibly windy location, so staking the trees will also be important.
https://www.gardeningknowhow.com/edible/fruits/jujube/growing-jujube-trees.htm