Complex sentence: After the battle, Julian was acclaimed as "Augustus" (co-emperor) by his troops, but he vehemently refused the title, which could only legally be bestowed by the ruling "Augustus", Constantius.
Split into simpler sentences: After the battle, Julian was acclaimed as "Augustus" (co-emperor) by his troops. But he vehemently refused the title, which could only legally be bestowed by the ruling "Augustus", Constantius.