the kit is made of 2 products, each with a different tax rate; this is what our suppliers are billing to us  and this what we need to invoice/bill to our own customers. Odoo is not managing this use case!

I am not sure but you may have to set correct tax on main product according to components. At the time of Product selection you have only info about what is inside the Kit(just information that may have you to add manually). On delivery order you can know what is to deliver.