My name is George and I am part of the Customer Service Department at IDX.
Thank you for your inquiry and I will be able to further assist you with your questions.
In relation to checking the fuse, it will depend on which charger you are currently using.
If you are using our most current NP-1 style battery charger, the JL-2PLUS, you can check the fuse by propping out the fuse box on the back of the charger.
The fuse box will be located in between the AC input and the power switch.
The fuse box can be opened/pulled out by using a flat-head screw driver and pulling the box out via the small indentation/tab on the side of the box.
Also, in relation to charging two batteries at the same time, as Eric previously mentioned, if the unit you are currently using the JL-2PLUS charger, the JL-2PLUS is a sequential 2-channel charging system.
Therefore, it would only be able to charge one channel/battery at a time.
Please let me know if you have any further questions.
If you would like, you can also reach me directly at my email: George@idx.tv