Why a double fuse is used in some applications?
Where is the safety ground applied?  In some utility battery systems the earth connection is applied in the middle of the stack.  As such, if the positive bar develops a fault to ground the current will circulate only in the upper half of battery stack.  Likewise, if a ground fault develops on the negative bar the current will only circulate in the lower half of the battery stack.  As such, both positive and negative need fusing.

In this picture below the loads are connected between positive and negative buses (250V).  When you picture a ground fault occuring on either bus you will see the path and need for fuses in both.



If your supply is truly ungrounded, you have the potential to develop a ground somewhere in the supply. In that case, it would be equivalent to what I describe above.