A search like this : user1 user2 or user1+user2 is enough afaik.
The default operator is "containing both" i.e. +
Source : https://developer.twitter.com/en/docs/twitter-api/v1/rules-and-filtering/search-operators
Seems to be working fine and I open your tweet, and I see the parent tweet.
But on opening the users tweet indeed it can't be seen. It can be on a variety of factors. I answered a similar question 7 years back. Have a look.