The first step to security is to have no illusions.
Qubes OS is great for data security but no technology per se can ensure social security (even in the so called civilized countries).
Do you think that keeping Qubes OS installed on the separate disk apart from main Linux distro is a good idea?
Only if you just want to play with Qubes until you get used to it.
If the answer for that question is no, what do you think instead about installing Tails on some pendrive and upgrading it regularly and having Tails for bad times?
I don’t know what bad times means, it is not a clear threat model. You can run Tails in a qube.