How to get posts from multiple categories ?

Hi, I am trying to get posts from different categories using category id’s like this
https://website.com/api/get_category_posts/?id=4+7
or using category slugs
https://website.com/api/get_category_posts/?slug=cat1+cat2

but I am getting results for the first category id if I use id=4+7
and error message if I use slug=cat1+cat2
{“status”:”error”,”error”:”Not found.”}

Is it possible to get multiple categories using JSON-API?
If yes how can I do that?

Thanks
Said