CiviCRM WordPress shortcode embedded within a tab shortcode does not load
CiviCRM can only be "invoked" once per page load, so multiple shortcodes on a page won't be functional. Instead they should display as "teasers" with links to the functional CiviCRM content on your WordPress basepage.

In short, you can't have functional CiviCRM forms unless they are they are the only content on a page.

There are a couple of ways you could do this, however:


use the Caldera Forms CiviCRM plugin to embed two forms in your tabbed page
pull in the content of the second tab in via an AJAX request


Personally, I'd ask myself whether implementing tabs is worth the extra development time.