You should each contact your relevant insurer and enquire how much of an additional premium will be required to add an extra driver to the respective policy for the week, when you have the info choose the cheapest. This kind of thing is done all the time by garages etc when arranging to give a replacement motor while repairs are being carried out (temporary transfer) so I wouldn't forsee any difficulty. Presuming you are of similar age, driving experience, claims history etc there shouldn't be a big premium involved, mainly just a charge to cover admin. I would have thought.