Filtering questions from multiple categories

Hi, 
thank you for this great plugin.
I’m trying to filter questions based on user’s subscription to specific categories, here’s my short code usage:

[dwqa-list-questions category=’some_category’]

 
this works well, but if I try multiple categories like this:

[dwqa-list-questions category=’some_category,second_category’]
I get no results

Can you please explain how to go about it?
 
thanks in advance!