Well, to combine clauses in a where statement, all you need to do is:
var data = from c in db.database_data
where c.UploadDate == yesterday &&
c.Cover == cvr
select c;
However, you might find that you get an issue with testing against UploadDate. The value you are storing in yesterday includes the time portion, so if UploadDate is actually a date, use
c.UploadDate.Date == yesterday.Date
in your test.