Solar Wire Size..??? Fuses on panels...??

The lower the DC Voltage the larger the wire has to be. In parallel you have to size it for 18VDC and add the amps from both panels. If you wire them in series you can use smaller wires as it will then be 36VDC and add the amps for one panel. There are reasons for either installation method.

I have 4 24VDC panels wired 2 in series & 2 parallel making it 48VDC. That allowed me to use 10 GA wire.

Go online & search for DC wire sizing calculator.