Is a constraint that provides referential integrity for the data in the column or columns. FOREIGN KEY constraints require that each value in the column exists in the corresponding referenced column(s) in the referenced table. FOREIGN KEY constraints can reference only columns that are PRIMARY KEY or UNIQUE constraints in the referenced table or columns referenced in a UNIQUE INDEX on the referenced table.
So what you could do instead is create a _temp1 with a primary key on either one of the two fields. Then create a unique index on both field1 and field2. When creating the table _temp2 reference to the primary key of _temp1.