The structures 除非…，否则… and 除非…，不然… are used in the same way. They mean “unless...”.
These structures indicate that the result "B" can’t occur under condition A. Under any other conditions, “B” would occur. The result usually has a negative connotation.
A sentence with this structure usually has two parts. The subjects are placed after 除非 and after 否则/不然.