I prefer separate systems, although they require more direct management than just a simple crossover pipe between the two tanks (ultimate KISS). With a crossover pipe between the two tanks and all the supply/return lines
from one tank only, the fuel
can transfer to the leeward (low) side when the boat is heeled. This can cause fuel
starvation while motor sailing if the fuel
level is below half and all the fuel
transfers to the "storage" tank. You can control this with valves on both sides of the crossover line
(which should be installed at each tank so you can shut them off in the case of a leak in the crossover), but that requires management and access to areas that are generally difficult. If you simply run a redundant set of supply/return lines
from the second tank to your dual racor, with a set of valves and "T's" you can select either tank and simply monitor levels in each. That is exactly the system that we have and have found it easy to manage, with the inherent safety of redundant systems.