Complex sentence: Jackie Washington had attempted to call his co-defendant as a witness, but was rebuffed by Texas courts because state law barred co-defendants from testifying for each other, under the theory that co-defendants would be likely to lie for each other on the stand.
Split into simpler sentences: Jackie Washington had attempted to call his co-defendant as a witness, but was rebuffed by the Texas courts. Specifically, the Texas law barred co-defendants from testifying for each other, under the theory that co-defendants would be likely to lie for each other on the stand.