I'm not a guru but I do have a similar setup and have been through these questions recently. 
The fuse(s) are there to protect the wiring, they don't limit the current except by blowing the fuse. Since everything downstream of the main battery fuse is wired to a breaker or fuse of its own if the demand exceeds the safe capacity of the wire the circuit should break before the wire melts. For example if you use an inverter and plug in a toaster and vacuum at the same time the breaker will open.
The battery fuse is for catastrophic overloads like dropping a wrench across the battery terminals. Without a fuse the length of the wire would burn possibly taking the fiberglass boat with it.
I have two 300 amp batteries wired in parallel with 1/0 AWG, and a single 300 amp ClassT fuse. The positive lead goes to a cutoff switch, then to the house bank switch. I switched that from 1/2/BOTH/OFF to a simple ON/OFF.
From there the positive goes to a buss bar. The negative wire goes to a Victron Smart Shunt and then on to the negative buss bar.
Two batteries wired in parallel are effectively one big battery and only need one fuse. Check your battery cables for size and condition or change to heavier wire and fuse for the wire capacity. 300 amps will work, less would work but might blow if you add an air conditioner, inverter, hot tub, etc.
One issue with the lifepo4 is it works so well many of us keep pushing the limits and adding devices. I have lots of computer gizmos, starlink, a Mr Coffee in regular use, an electric kettle and induction cookplate for occasional use. Much of it runs off a Victron multiplus  3000 watt inverter charger. There are 3 200 watt solar panels wired in series and a DC to DC charger for use when the engine is running. Once in a while if I'm not on the move or in several days of rain I'll need to break out the small generator or plug into a slip. 

I am new to the group having been a C30 owner previously.
i am not a guru or expert and I ask anyone to correct me if I am wrong:
Fuses are there to protect the wire in case of a short circuit not the devices at each end. They are not resistors that 'throttle' the power delivered.
it is generally accepted that you put a T class fuse within 7" of the lithium battery. From there, there is nothing wrong with using a 2 or 4 ga wire on the lithium as long as it is feeding  devices that don't draw more than 150 ish amps (inverter?)- as long as you fuse the wire with an appropriate fuse for the wire - say 200 amps.
in other words... use the wire gauge appropriate to the load you are putting on it and with regard to keeping the voltage drop to less than 3%. I hear the Epoch's are well regarded - they must have very beefy internal BMS's to handle 300A continuous. I built my own battery for my old C30 with EVE 280ah cells and an Overkill BMS rated at 120A continuous. I can't remember now if I used a 200A T class or something around that rating. I'll probably use a 200 to 250A T class fuse on my new boat when I move the old lithium's over.

you can learn a lot through Mainesails site, Pacific Yacht systems and Emily & Clark's adventure on YouTube.

Gerry Davies
Amara 1995 C36mkii # 1392
Tampa FL
 

I somehow missed seeing Brazen's response to your question - my experience and understanding match his. He has a nicely laid out system. I note he has a separate AGM starter battery which is appropriate.

If you have access to the settings of the BMS you would do well to try and top balance the cells in both batteries before paralleling them to get the most capacity out of the combination before a BMS shuts a battery down because one mischarged cell has hit the high or low voltage 'knee' prematurely. Will Prowse on DIY SOLAR YouTube channel and forum has a great explanation of how and why with balancing.

Gerry Davies
Amara 1995 C36mkii # 1392 TR/ WK/ M35A