why are two subs harder to place than full range?

Actually- especially for the novice - I wholeheartedly recommend keeping the subs up near the main speakers, especially in systems with adequate room DSP and/or Parametric EQ's, ideally. In these cases, it is most often much easier to get a coherent sound, seat to seat, with subs placed up near the mains - I find.
Yes, you need to consider the output capabilities of the subs and the volume of the room. If your sub/volume ratio is on the short side, you might consider close corner proximity for the subs, with some play. Still, if they have the output, and you can "EQ" out the response of the subs somehow, you have the strong advantage of not running into as many PHASE issues between where the mains leave off and the subwoofers pick up - among other things. And, less than ideal "modal placement issues" can be EQ'd out, as long as you're not setting the subs in "holes" in the response curve.
Basically, I like the blending ability and coherence potential of subs near mains, personally.
If you put one subwoofer next to your seat, sidewall, etc, and you have multiple seats aroudn the room, you are most assuredly running into phase issues from speakers, to subwoofers, to seating possition, ideally. I don't like this tradeoff personally. Also, If you start putting subs at 1/4 points around the room, you are DEFINILTEY starting to deal with "phase cancelation" and comb-filtering issues, most often. And this is also hard to deal with, if you're not careful and know what you're doing.
My rule here is to try and "keep it simple &*#$@%!" That way you have a better chance of well balanced, dynamic, coherent sound.
I once did a system for a guy who had a couple of monster high end subwoofers, set up mostly out of phase with each other, and the mains! He complained of "no bass" in the system, even with the subs cranked up. I placed his subs up near the mains, right next to each other, off center of the room slightly - and it sounded great, but he didn't like the look. We ended up with mostly the same performance with one sub out of phase, placed in the corner, to good effect.
Hope this helps

You can EQ a room to death and still have bad sound.
Take the case of a room with dimensions which are even multiples of one another.....say 8foot ceiling with 16x24 foot floor size. NO amount of EQ will get you away from all sorts of standing wave problems.
HSU (Dr.) recommends near-field sub placement. Right next to you, if possible!
The Harman White Paper recommends 2 subs, placed assymetrically. If anyone is interested, I'll find the link.
My personal listening space is a very odd shape. 8 walls, some at a 45, and a vault ceiling, also not symetric. My sub is near the mains, up against a side wall (the longest wall) and about 4 feet from the corner. Downfiring sub, with the port facing backwards. Decent integration, and with the X-over at 40hz, quite seamless with my Maggies. The room is well over 5000ft3. This is the right way to measure a room, since for any given floor space 8' vs 12' vs high vault ceilings will make quite a difference.

iplaynaked offers the best first cut for my situation, and i appreciate the advice. if that doesn't seem to work i'll wait for my back to heal and start moving some furniture and listening to the sub in different positions. btw. my room is about 14'x 23' with 12' vaulted ceilings. i'll be using a pair of tekton design fostex 167e single driver speakers on the short wall and two cheap aural ecstasy subs (i expect not to turn them up very much) listening at approximately 9 feet away. thanks again everyone.

magfan, in my last apartment that is almost exactly where i had my, at the time only one sub, and in the much smaller room i had huge bass with almost no volume (xover @ 60hz) and what seemed good integration (at least it didn't seem to take away from the sound of the tektons running full range, just an addition of lower frequencies).

First get a dolly so you can easily roll it around. In most rooms, deep bass does not couple in the same plain as the mids/highs. If you notice, most really expensive speaker systems have separate subs and a XO

If you use one sub, try to place it on the right side. NOT IN A CORNER.

The wringing of the hands is worth it

GOOD LUCK

Whatever works in THAT particular room.
/ listen / move sub / listen some more.
repeat until satisfied. Than leave it alone
for a while.