I planted these two red buds at the same time about two years ago. The one on the right is doing great, but the one on the left has been looking terrible the last few days.

Just like people going to the doctor, sometimes a little background information can assist in a possible diagnosis so perhaps listing a few of the following?

General location?

Water intake? Sprinkler head nearby? Has it rained a lot? Pooling water? Etc

Fertilizer?

That type of info can help a lot.

Edit to add-for a young tree, it does look like it is having to compete with some other plants nearby-have you tried a dose of fertilizer high in nitrogen? The older leaves are a bit yellow, but I'm not seeing a lot of new leaves, FWIW.

I'm in San Antonio. We did get rain this weekend, but not a crazy amount of water. It usually gets fairly consistent water. I used fertilizer spikes for both redbuds in the spring.

From Google, it seems like it has some symptoms of Verticillium Wilt.
https://www.treehelp.com/redbud/

Quote:

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The first signs that a tree has a Verticillium Wilt infection is the yellowing and then browning of leaves at the ends of some branches. Initially the yellowing and browning of the leaves is spotty throughout the tree and does not follow a uniform pattern. As the fungus begins to block the vascular system, the browning of leaves becomes more acute and more wide-spread. New leaves generally are either non-existent, undersized or yellowed.

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That does seem like it could be. Hopefully you can save that one and/or keep it from spreading to the healthy plant.

I had a redbud that looked pretty similar to this a year ago. The limbs with yellow leaves ended up dead, so I waiting until winter and trimmed off all the dead limbs (which was probably over half of the tree). This spring the rest of the tree bounced back and looks really healthy now.

FWIW I think my problem originally was from not watering enough last summer but I could be wrong.

On second look, it's pretty much every limb on yours so this may not be good advise.