Using module_invoke() and render() causes PHP warnings
Blocks aren't render arrays, they're made up of a subject and content key.

You'll probably have better luck referring to those explicitly:

$content_search = module_invoke('search_api_page', 'block_view', 'content_search');
$user_search = module_invoke('search_api_page', 'block_view', 'user_search');

print render($content_search['content']);
print render($user_search['content']);