Racor makes different multiple filter models, some will allow you to isolate one filter and run on the other so that you may change a filter without shutting down an engine, and others will not allow you to do so.
It is important to ensure that the filter system is rated for the flow to your engine. And equally important, the flow to your engine is *NOT* the same as your gallon per hour fuel burn! (Diesel engines suck a lot of fuel, then return what they dont use to the tank)
The newest Racor filters, the MAX series, are very nice in that they have a single selector valve that isolates the filter allowing you to change one while keeping the engine running on the other.
I change my Racors at every oil change (100 hour interval), I figure its far easier to change them at my convenience rather than waiting for them to clog up.
Also, bear in mind there are 3 different filtration levels I believe they are 2 micron, 10 micron, and 30 micron. if you have any contamination at all in your tanks the 2 micron will clog very quickly; I use 10 micron, I figure the secondary filters on the engines will take care of anything the 10 micron filters dont catch