Have been waiting and working around this for years now. Please consider/accept this as a possible and simple solution for this problem:
Control/Command+Enter on a query in the search bar should behave as it does in other contexts, i.e. open a new tab without switching focus to it (background tab). This will then allow us to simply navigate with keys to another search engine and press Enter again to achieve the same and so on thus resulting in many search tabs being open for a single query string.
Incredible how long we can survive without some crucial features instead of getting involved - had to look into this now already and offer my thoughts. Thank you! _/_