This reply has been accepted as the best answer.
You have to duplicate the whole selector
.is-mobile.page-id-xxxx #primary-widget-area, .page-id-yyyy #primary-widget-area {display:none;}
If you have a lot of pages, a better way is to give all these pages a common class like
myhidebar
that you would place in the Page editing page > Xtreme Options > Xtreme Plus > per page added classes > Container box
Then the single rule below will affect all the pages with that class
.myhidebar #primary-widget-area {display:none;}