Have a read of this thread.
Just bought another CAV filter fo £39 from our local diesel injector factors, for use on an Optimax. Spare cartridges are £1.20 from ASAP Supplies.
If you connect two fuel tanks then only one may draw until empty leaving the other full. Easiest way would be to switch over the fuel tanks when one gets low, you could do change over valves but why not just keep it simple. Carrying a spare fuel line from tank direct to engine would be good if it all goes pear shaped.