When I put in two wheels (18x9.5, or 17x9 (same rolling diameter with tyres on)) into the back of the 33 with rear seats in, it pushes against the trim panels squeezes them in about 1cm... So two wheels car sit side by side just fine and then two more stacked on top...
Not sure how they'd go standing upright (same orientation/direction as if they were on the car), but I don't put them in like that in case I need to brake hard
If I'm just putting two wheels in the back seat, I normally have one sitting flat, and the other sitting on an angle on top of it. Doesn't push against the side trims that way.