Is this doable ? and is it a good idea ?
 

I was thinking of ing the audio spectrum across all my speakers in the car.

I.E. 6½ s for highs, 6X9s for mid-bass and sub for bass
so, something like 2000 ~ 20000hz for the 6½s, 80~2000hz for the 6X9s and 80 and under for the sub

Good idea ? Or not ?

That's pretty much what you're supposed to do, but I'm not sure about the frequency you should set the crossovers at for 6.5" speakers or 6x9's. For the subs, try setting the crossover at 60 or 70.

I'm not quite sure about the frequencies myself, and I won't really be using crossovers as each speaker is on a seperate output from my amp. I only have a single 10" sub in the car.

Is it possible to make some sort of a filter that doesn't split the frequencies but rather eliminates them ?

I can have a LPF or HPF from my amp with a 40~400hz range.

My thought is I can use that to filter out the lows, but for the highs on the 6X9s, I would need some time of filter, but not necessarily a crossover.

Also, if somebody could give me a better idea of how to split the frequencies among the various speakers, that would be great.

Why not just have the high's going to the 6"x9"s aswell. No harm in highs coming from them.
But other than that, get a inline crossover. It goes before the amp, in adjustable high, low, and mid. Then you just put the rca's into the inputs that you want the speakers playing.

A filter and a crossover describe the same device - something that restricts or removes part of the spectrum, whether passively or actively.

Get yourself a decent 3way electronic crossover like a Coustic XM5 or XM6

If you want to keep it really cheap/simple, just run a small capacitor inline on each of your mids (www.bcae1.com for 'crossover' values).