If I buy a 2 day one park pass do I have access to Disneyland one day and California adventure the next ? Or do I have to stay in one park for two days?

You can choose which park you want to go to each day.

Yes, you can do that. However you cannot leave one park and go to the other without upgrading to a park hopper pass at $50/ticket.

You can go a day and a day if you paid the package for the two parks, no problem .... have fun