The difference in voltage is 0.2V - the Photive is 7.4V rated at 1700 mAh (or 12.6 Wh) versus the Canon which is 7.2V rated at 1120 mAh (or 8.1 Wh). Therefore the charge capacity is greater in the Photive versus the Canon. The handgrip would put both batteries in parallel so as to not exceed voltage going to the camera. I don't think combining a Photive and a Canon battery would be any different than using an older (weak) Canon battery with lower charge capacity and putting it in parallel with a newer Canon battery with optimal charge capacity - ultimately the newer battery will "carry" more of the load as the weaker battery loses charge. Because the camera doesn't require more than one battery to operate, practically I doubt whether this configuration should cause any problems. Note however that in most battery manufacturer websites and in practice, it is a recommended practice to not combine batteries of different types nor ages. Given the low cost of the Photive batteries, I would simply buy two Photives and load then into the hand grip.
I'm not a battery expert, but I do have a degree in electrical engineering. There is a possibility that the OEM battery will drain power from the aftermarket battery. Just like you can use a portable battery pack with a USB output to charge your phone, a higher voltage battery connected in parallel to a lower voltage battery will transfer charge into the lower voltage battery until it is full. Then, some batteries will continue to allow current to pass when an overvoltage is applied, and in this case any remaining charge in the aftermarket battery would be wasted trying to continue to "fill" the lower voltage battery, and continuing in the metaphor if the OEM battery spills excess charging current "on the floor", then this would be the outcome. The speed of that spillage would be relatively low, but could happen even when the camera is powered down (if the batteries are not isolated from each other in the powered-down state). I agree with one of the other commenters: better to just buy two of the aftermarket batteries and use them together.
I would be very careful using the Photive battery together with a Canon battery in a battery grip, as the two batteries are rated at different voltages. You may not have any problem, but it might ruin one of the batteries.