Condition for Assets Tag

I'm trying to loop through the same slider markup using two different asset variables. Is there a way to do this? I already tried a few different approaches but didn't find a solution that works.

I'm trying to do something along this line:

{{ assets:images or assets:post_intro-images }} 
{{ /assets:images or assets:post_intro-images }}

I tried this as well. Didn't work either:

{{ if images }}
{{ assets:images }}
{{ elseif post_intro-images }}
{{ assets:post_intro-images }}
{{ /if }}

{{ if images }}
{{ /assets:images }}
{{ elseif post_intro-images }}
{{ /assets:post_intro-images }}
{{ /if }}

Anyone has a solution on how to approach this?