It depends very much on your attitude and how thorough you need to be.
Yes, you can decently cover both parks on lower crowd days provided you're willing to forego some attractions if necessary.
- Be sure to get familiar with the FastPass system.
- If you stay at a Disney hotel, you get Extra Magic Hours where they let hotel guests in ~2 hours before regular opening.
- Walt Disney Studios us much smaller so if you do go for both, go there first, see what matters to you, then spend the rest of the day at Disneyland.