Try to delay it by a week saying you have some emergency that you can't avoid? If that doesn't work, I would personally start working in the first job, and still go for the second interview. If you get the second job, just quit the first job but very much easier said than done.
When you quit your first job, be open and honest - second job is a dream company and with the pandemic and everything, you thought it would be the best if you started first and be honest about it after. It won't be a good outcome and first company will definitely be annoyed - but hopefully they understand from a personal standpoint and I'm sure they would have done the same if they were in your shoes.
Not an easy decision & situation to be in, all the best and congrats on starting your career! You will never forget the feeling of having your first pay check (and then losing it immediately through all the online purchases you do right after lol)