Hi,
Well, if you want two fields, then you should create two separate search bars, and configure one to search the posts, and the other one to search the comments only.
Another possible solution could be to use content type filter, that is visible below the search bar, so the user can choose what to search, that looks something like this: https://i.imgur.com/qtqNS4Z.png
For this feature, you can follow these steps:
– Enable the search in posts and comments: https://i.imgur.com/mSDAWCI.png
– Enable the filters box to be visible at all times: https://i.imgur.com/ZVtI8km.png
– Use the content type filters: https://i.imgur.com/BJZ0ocg.png
I hope this helps!