Both tawriyah and istikhdām involve more than one meaning, but their mechanisms are distinct. Tawriyah first evokes a near sense while intending a remote one. Istikhdām assigns different senses to an explicit word and its pronoun or to two related pronouns.
FeatureTawriyahIstikhdām
Diagnostic clueNear/remote sensesPronoun reference
Intended effectRemote sense selectedDifferent senses in different positions
A shared lexical form alone cannot establish istikhdām. The semantic shift through a pronoun is necessary.