I asked an electrician at work and found this out. I'm kinda slapping myself because I knew this in the first place.

Two 30 amp fuses wired in series = 30 amps
Two 30 amp fuses wired in parallel = 60 amps

The fuses in the aforementioned holder work in parallel. So two 150 amp fuses = 300 amps and one 150 fuse = 150 amps.