I've found that I just can't water my hanging baskets often enough to keep most allegedly "drought tolerant" plants looking good, I hit the success button when I converted to succulents.

You're not fond of the crown of thorns but admit that it blooms like crazy in your conditions. Instead of getting rid of all of them, how about leaving 1 or 2 in place (depending upon the size of your window box), and adding one of the lime green sedums that seem to be easily found at even the big box stores (pictured below).

Then think about a third plant that would offer contrasting form/color, etc. Consider the paddle plant (kalanchoe thyrsiflora) named "Fantastic". In full sun it displays a great red color on the margins (picture in the next post)

The red of the crown of thorns and Fantastic would echo each other and the lime sedum would provide some zing. The foliage of all contrast nicely with each other and you'd get some height variation as well.

June

This is the kalanchoe "Fastastic":