Available from most motor factors and some engineering groups. We have twin filters set up for both engines, very handy to just throw the change over valve if-when the filter blocks or is restricting flow, change of old filter can then be done at our leisure when in port.
I just tee'd off the fuel supply with an on-off valve before and after each filter, then tee'd back in. The filters, valves and pipe were fitted to a board so I didn't have to try and do it all in situ, then the board was screwed to a bulkhead alongside the engine.
To turn it into a 2-filter system you would need to make (see your last diagram) valves 1&2 three way valves and place a filter in the position of valve 3?