Yes, they can both go to ground. Either on the blend, or any other convenient place if it's too tight on the blend.
Be aware that there is a right and wrong way to wire up a blend pot. They are not symmetrical. If you have a meter, put the blend at the center detent, and then take a reading from wiper (center) terminal to either outside terminals.
One will real almost zero, and the other almost the full value of the pot. The low value side is hot, and the high value side is ground. The other deck will be opposite.
If you wire it up backwards it wont blend right.
Follow this chart I did, but it might not match your blend pot. these were the readings on a Bourns M/N taper pot.