My question is that we as shia read namaz as zohrain and maghribain which is allowed, but on the day of Ashura, Imam Hussain a.s offered zohr and asar prayers separately. So we get more sawab offering them separately? and when we're offering namaz as zohrain or maghribain so what are the times that these namaz get Qaza? if we offer zohrain at the time of asar so the zohar namaz won't be qaza right?

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.

My question is that we as shia read namaz as zohrain and maghribain which is allowed, but on the day of Ashura, Imam Hussain a.s offered zohr and asar prayers separately. So we get more sawab offering them separately? and when we're offering namaz as zohrain or maghribain so what are the times that these namaz get Qaza? if we offer zohrain at the time of asar so the zohar namaz won't be qaza right?

Its actually better to make them together because you’re making them at their earliest time. If you separate them then you are delaying the second prayer. Imam made them separately in this occasion due to a reason that necessitated it.