Get posts from two categories

user this method
“category__in[]”
https://domain.com/wp/api/get_posts/?category__in%5B%5D=8&category__in%5B%5D=9

This works for me ??

I did not find category__in to work. If I put in a list of values, only the first one was read, if I put in the query var twice: category__in=8&category__in=9 – only the second one was read, i assume overwriting the first.

But I was able to use a different query var on the get_posts call to make a multi-category query
https://domain.com/api/get_posts/?post_type=customtypename&cat=17,15

I used the above to query a custom type – costumtypename
and use cat=12,13
where the values are the category IDs, separated by commas