Sorry Jean, I am going to solve my own mystery again
I was wrong with not being able to have more than one collider per object. I just didn't pay attention to the other options in the replace collider message window.
Anyway, for those who are interested, in my cast I have a tower that uses a trigger sphere to be aware of enemies entering the trigger sphere collider.
I also wanted to be able to target the tower with mouse pick but not pick the trigger sphere collider.
The solution was to
- disable the Raycast Hits Triggers option in physics settings
- add a non trigger collider to your object (mesh or basic shape collider)
And now it works like a charm, perfect.