Question

They had been stitching the dress for two hours in passive voice​

Answer 1

Answer:

This sentence is in past perfect continuous tense.

Any sentence that is in past, present or future continuous tense, cannot be changed into passive voice. So, there will be no change.

Hope u can understand!!!

Answer 2

Answer:

The dress had had been streched for two hour

Explanation

Had been is already a passive , but if u want to change it forcely, then use had been+past participle