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They (Hermione and Ron) did not stop arguing all the way down to Snape's dungeon, which gave Harry plenty of time to reflect that between Neville and Ron he would be lucky ever to have two minutes of conversation with Cho that he could look back on without wanting to leave the country.

_{Harry Potter}

I don't understand "between Neville and Ron" in this context. Can someone help to explain it in the context?

Compare:

Between school and my job, I don't have much time for anything else.

The phrase means "given the combined (encroachments or demands of) school and my job" there is no time left over for other activities.

So, in your example, it would be "given the combined (encroachments or demands of) Neville and Ron" a good two-minute conversation with Cho was unlikely. The nature of those encroachments or demands is supplied by context. For example, it could be that the two of them, whether individually or as a duo, were monopolizing Cho's time, or monopolizing Harry's time, or that they were goofing off with each other so that there were constant hijinks and never a quiet moment where Harry could talk with Cho. Whatever. You'd have to look to the wider context in the book to get a sense of how Ron and Neville were interfering in this.

According to the Oxford Dictionary

between [preposition]

By combining the resources or actions of (two or more people or other entities)

I didn't remember what's happening exactly at that point of the book but it may mean that if you add [combine] the time that Harry spends with Neville plus the time that Harry spends with Ron [two people] then he would be lucky ever to have two minutes of conversation with Cho.

I want to remark that the relevant part in relation with your question is combine whatever they both are doing. What they are doing exactly is not present in your excerpt.

Probably in the context of the book the whole sentence is not about Neville and Ron wasting Harry's time but they are combining their actions somehow to prevent Harry for having a conversation with Cho.

In the context of the book, if memory serves, Cho came upon Harry while he was sitting with Neville on the Hogwarts Express just after Neville's Mimbulus mimbletonia exploded, covering Harry in stinksap, which he found embarrassing. Later, Ron angrily challenges Cho on her support of a Quidditch team and picks a fight with her, also embarrassing Harry.

In your quote, Harry is saying that because of Neville and Ron combined, it seems that every time he tries to talk to Cho, he is embarrassed. It is not necessarily implying that Neville and Ron are working together to embarrass him (or that they are even doing it intentionally) but that their combined actions are making it impossible for him to have a positive conversation with Cho.