See my response post. We can't make trade deals with any country. It has to be done by the whole EU for the whole EU.
We can TRADE with countries the EU doesn't have deals with, but if we wanted to amend terms in a way which we and that country think would be beneficial, we can't.
So we can make trade deals with countries EU doesn't have trade deals with yet can't make trade deals the EU has deals with. But if we already have a deal with a country in the EU setup why would we then setup a separate deal outside the EU setup with the same country? Undercutting the EUs deal in some way to gain some kind of advantage?