Lets say you want to target two different URLs, and only mobile visitors on those pages.
In the example image above we target two start pages (one for english visitors and one for french visitors) and also only mobile visitors for these sites.
What you need to do is to set the statement with an OR between them in an own If block. And then add an own If block for the device statement. This is needed to get the statement logic correct.
The Audience in the image above reads (notice the parentheses):
If ( URL Simple match https://www.example.com/en OR https://www.example.com/fr AND Device is Mobile )
If you set this up in the same If block the Audience would read:
If ( URL Simple match https://www.example.com/en ) OR ( https://www.example.com/fr AND Device is Mobile )
This would make Audeince TRUE if URL is https://www.example.com/en no matter deveice, which is not what we want.