I haven’t looked up the amp draw of an OB2, but let’s say they each draw 20 amps. I agree with the 35 amp fuses for each compressor, but the fuse on the 1/0 should be bigger. It needs to be big enough to handle both running compressors. So if they draw more than 40 amps together, that 40 amp fuse will blow.
Actually, fuses are meant to protect the wire in case of a short. You want the fuse to blow before the wire melts & causes a fire. The deciding factor for sizing a fuse is the rating of the wire gauge -vs- its length. There are online charts for amp ratings of wire.
In general, even if it’s cheap 1/0 and 16-20 feet of run, you could probably use a 150amp. If you want to be overly cautious, you could use a 100amp. It needs to be close to the battery, so the majority of the wire is protected.
Any time you reduce wire gauge (such as a distribution block) you need another fuse that is sized for its gauge and length. If the OB2 has like 2 feet of 10 gauge, 35amp is good.