Why didn't the Battlestar Galactica have more than one FTL unit so that it could jump and rejump without any delay? That would seem very useful in battle to jump in, fight, and leave within minutes. Better yet, jump in, launch missiles, jump out, jump back and keep doing that until the base stars retreat. Obviously, this would spoil the fight sequences, but it does seem like a good idea to have on the larger ships or at least one specially fitted raptor.
I believe the time-frame between jumps was governed by the computers coming up with a jump solution, rather than needing to charge up.
And the calculations could not be done until you arrived at the location for the next jump.
A second jump system wouldn't help with either of these points.
As well as jumping being limited by computational requirements (which aren't all that bad when you look at episodes like the rescue of New Caprica - at one point the Galactica jumps just long enough to launch vipers before jumping again to engage the Basestars), we see in Season 4 that the FTL drive is really frakking huge on Galactica.
Obviously, the Raptor FTLs aren't anywhere near as big, so it would seem larger ships need larger drives.
So, for the Galactica to be able to have a second drive, it would have to be much bigger than she already is - which then means the two FTL drives would have to be bigger just to move the bigger ship (which now has to be even bigger to fit the now larger drives).
Obviously, the Colonial Fleet decided to expend resources on lots of smaller vessels with single drives instead of fewer larger vessels with redundant drives for not much gain.