How do I get a double top boostrap nav with bootstrap template? I want a secondary bootstrap navbar on my site, a black one on top of the blue one.

Now when resizing, there is a hamburger menu and so on, so stacking 2 bootstrap menus on top of each other, even of different colors may cause problems.

How do I have a double bootstrap menu with 2 colors?
You can create a region in yourtheme.info or use Block Group Module.
If you're using Bootstrap theme for Drupal, you can simply write code for your Top Links Navbar in page.tpl.php and use CSS to design it as you wish. Now in the template code, include your navbar if not empty:

if (!empty(primary_nav)||!empty(top_links)||!empty(user_nav)||!empty(navigation))
{ //..Code to include your top_links navbar .. /}


You can also hard-code your menu in page template of your theme but it's not the best practice.

Plus: Always use the child theme.

If you want to add hamburger menu then use JQuery pluggin for same and remove the default responsive behavior of bootstrap from the menu you want to display hamburger on smaller screen. I advice JQuery plugin as it's open source, clean and very beautiful. 

Play here.