The 7450 has 3 module slots, if there are 2 x 40G modules in the rear slots then there is only the front slot available for a 10G module.
If you have two units stacked using the rear 40G ports then you can put 10G modules in the front slots for a total of 8 10G ports across the two switches.
An alternative configuration would be to put a 4x10GF module in the front and use that for stacking and that would leave space for 2 10G copper modules in the rear of each switch.
There are a few options so I would recommend checking the hardware installation guide as all the details are in there.