Rich, do you thing that 2 filters in series is a good idea?
In a delivery system that has symptoms of gross contamination ....
definitely a prefilter (@ approx 5 times the retention size) will
VASTLY extend the life the final filter.
If you have an active independent recirc. polishing (any kind) then
probably: no .... as the challenge to the delivery system filters will
approach zero. But a belt and suspenders approach is always good for
safety .... your decision.

If so then how
about vacuum
gauges? Shoufl I install one in between the 2 filters?
MOST DEFINITE YES!! The ONLY way to monitor filtation operation
performance is by gauges ---- then you can monitor WHEN to change the
filters. But you should periodically also record the pressures to see
which one is in need of replacement. What you will develop is the
state of the present entire system (history) versus engine hours ...
and then in future change before you reach a critical point .... or
ascertain that the system has degraded by bacterial contamination,
onload of bad fuel, etc. or worse.... if you see that a value has
*decreased* or is less than previous, then you know to change a filter
that has a *hole* and is now bypassing.
Without gauges .... you just wait until something inevitably "chokes"
.... or needlessly throw away a lot of filters with lots of life
remaining.

If you do have TP filters already installed, put a gauge across them
and watch for the differential pressure to vary from day to day ....
indicating bypass or unloading.