mixing subs

One thing is that you want to make sure you test them individually as well as collectively to find that they are all hitting at the same time.  Polarity can sometimes get confused in a multidriver setup.  It's also best to have them all aligned in the same direction, although you sometimes see showcars with the subs pointed in all different directions.  When they are all nearly aligned along a single plane, seen as straight across, the waves are more likely to reach the target at about the same time.

A popular thing to do for the past few years has been to invert half the subs and leave the other half firing normally.  You can do this for a more unique look or to make better use of available space.  The inverted subs, seen as showing the magnet, are wired in reverse so that all subs fire in one direction at the same time.

Otherwise, your question "how should they be installed" is too open.  Use this thread for more specific questions than that.

what about this setup? Just curious, if you had 2 1000watt mono amps and you have 2 12" subs at 500watts each, 2 15" subs 500 watts each.you wire both 12's to one amp and both 15's to the other. Lets say that both have seperate enclosures tune to 35hz how would that sound? and what if one enclosure was tuned to 30hz and the other to 40hz how would that sound?

If you want the 12" subs to play the higher freq. and the 15" subs to play the lower would this be a good idea?

Different speakers reproduce sound differently.  If one sub is moving slightly slower or faster than another, you will have cancellation.  If you wanted to do different size speakers or even different model speakers you would want them playing different frequency ranges.

If you were wanting to do two 15's and two 12's for instance.  You would want the 15's playing something like 20-60hz with a steep crossover.  And have the 12's pick up 60-80 or maybe 60-100.  Again steep crossover slopes.  Definetely separate chambers for the different sizes.  Best bet separate chambers for each sub. 

Its not hardly worth the design involved to make this set up sound good.  Or be very loud for that matter.