I have not come across where Imam Hussain (as) offered them separately.
There is no extra reward in offering them separately.
We offer them together because it is best to pray them at their earliest time. Asr time is right after the amount of time that it takes salat ul zuhr to be completed. Same for Isha sets in at the time after maghrib would have time to be completed.
The time salat ul zuhr or maghrib becomes qadha is when there remains only sufficient time left to make salat ul asr and salat al Isha respectively.
I hope this makes it clearer for you inshallah.