Disneyland Resort has a variety of ticket combinations for visiting Disneyland Park and Disney California Adventure Park. The “One Park Per Day” ticket allows guests to visit one of these two parks per day. Should guests want to visit both parks on the same day, they would purchase their ticket with the “Park Hopper” add-on. When purchasing a Multi-Day ticket, guests may choose the One Park Per Day option and visit both parks on different days, however, that guest CANNOT use an extra day of the Multi-Day ticket to visit both parks on the same day.
For more guidance on purchasing Disneyland Resort tickets, look here: