Ref. No. 1448/42-900
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
Nafl prayers can be offered at any time, but if it is offered after two Rakat sunnat as mentioned in the question, it is better.
And Allah knows best
Darul Ifta
Darul Uloom Waqf Deoband