I suppose that it depends on which Filter'Tron we're talking about.
Real FT's (following the original Ray Butts / Gretsch recipe) are low DCR and low inductance humbuckers : around 4k and less than 2H. Their very weak coils are compensated by double thick A5 magnets giving more output and long screw poles introducing a bit of eddy currents to mellow the sound.
Lipsticks are even weaker, with an inductance around 1.3H - and more eddy currents.
I have both in different guitars and find their characters rather different (FT's being gritty and Lipsticks, airy). Not sure those that I have would cooperate nicely in a same instrument. But who knows and why not?
Side note - I can't imagine a
vintage style FT wired in parallel: it would measure less than 0.5H of inductance, which seems incredibly low for what is still a high impedance passive transducer...
Maybe "Filter'Tron" variations like those designed by Duncan have higher DCR and inductance than vintage correct ones and allow parallel wiring: I don't know, having no experience with hot rodded FT's.
Below are the electrically induced resonant peaks of a vintage correct Filter'Tron (series wiring) and a Lipstick, compared to the peaks of a Strat pickup and a Burns Tri-Sonic. Different animals electrically, even if they have all rather underwound coils...
FWIW. HTH.
