If you're running 2 subs off one mono-block amp, that may be the cause of the dimming lights.
Running 2x 4ohm speakers reduces the impedance to 2 ohms which many amps will stuggle to power and that will pull more power from the car's battery/alternator.
If your subs are 8ohms each, then the impedance will be reduced to 4ohms in total which can still be a bit low for some amps.
Do you really need 2 subs? Is the extra weight and bass volume really needed?
If you can't do without it, then I would definately recommend you run each one off it's own amp channel.
A power capacitor can only do so much. The alternator on the car may be a bit puny considering you've got the 1.4 engine.
Try disconnecting one sub and see if that helps, if not, try a power cap, if that doesn't help try an uprated alternator.
I would always advise running one speaker/sub per amp channel, it will always sound better.