Incorrect: «رَأَيْتُ طَالِبَانِ» — the object has been given a nominative form.
Correct: رَأَيْتُ طَالِبَيْنِ.
Incorrect: «مَرَرْتُ بِالطَّالِبَانِ» — a nominative form appears after a preposition.
Correct: مَرَرْتُ بِالطَّالِبَيْنِ.
Mistake: treating كِلَا الرَّجُلَيْنِ exactly like كِلَاهُمَا. Correction: Check what the word is annexed to.