From the MySQL documentation :
PRIMARY KEY
A unique index where all key columns must be defined as NOT NULL. If
they are not explicitly declared as NOT NULL, MySQL declares them so
implicitly (and silently). A table can have only one PRIMARY KEY. The
name of a PRIMARY KEY is always PRIMARY, which thus cannot be used as
the name for any other kind of index.
https://dev.mysql.com/doc/refman/8.0/en/create-table.html
If Field2 can be NULL, I question why you need it as part of the Primary Key since you then need Field1 to be distinct across all rows. So Field1 by itself should be sufficient as the Primary Key. You could create a different type of index on Field2.