I answered my question which was, I think, quite similar, by doing the following (I was not understanding well prestashop tax rules):
1) Create a Tax rule
2) Create a new rule, for country A, associated with tax 1. Set the behaviour to "combinate" or "one after the other"
3) Create a second rule, for country A, associate it with tax 2, and set the behaviour again.
==> Final price will either combinate the 2 taxes or apply one tax after the other. I think you can do it for as many tax you wish to.