Placement of dual subwoofers...

What are the choices when using dual subwoofers? Is it crucial that they be equa-distance from the listener like the mains?
I would say yes, if you don't want to ruin the rhythm and pace of your music. Placing subs in the corners in a high end system will result in more bass QUANTITY, but it may be a bit bloated and slow.
Dr.Gizmo Harvey Rosenberg wrote some stuff on this a few years ago, suggesting putting your subs on stands and having them in a near plane to the mains (you can probably google this article, good reading!)
I have adopted this set up with my Audio Physic Virgo speakers and dual Sunfire subs, with the subs just slightly behind andto the outside of the mains, and my system has never sounded better.
Where you place your subs depends a lot on their design and the design of your mains, as well as your reasons for using them in the first place. Consider where they are crossed over to the main speakers, the crossovers slopes, as well as room acoustics. If the crossover point is high and the roll off is shallow the subs will have a lot of audible stereo info and if not placed close to the main speakers will screw with your stereo imaging.

If you plan to run your mains full range and they go fairly low, like 40to50hz, and you have a low cross over point on your sub you may be able to place them most anywhere that adequately provides the additional bass reinforcement you are looking for. If your cross over is low and the slope is very steep you have more flexibility.

Personally, I'm sort of fussy about good stereo imaging so if I were to use subs I would always have them as close to the main speakers as possible unless there was a bass node or null creating problems in that area that needed to be delt with.

Have fun. Good sub integration can be a PITA.
Well you'll probably get as many suggstions as you get responses, if not more. For instance, I'm offering not one but two suggestions.

If you want to maximize the sense of spaciousness that subwoofers can give you, and if you're running them in stereo instead of dual mono mode, place one to the extreme right of the listening position and the other to the extreme left. This will maximize the interaural time delay between their two arrivals, and is the technique developed by David Griesinger of Lexicon. You can read his paper entitled "Loudspeaker and listener positions for optimal low-frequency spatial reproduction in listening rooms" here:

The second technique I recommend hasn't been formally written up to the best of my knowledge, but it's developer is physicist Earl Geddes. This technique aims to give the smoothest bass over the largest possible listening area. Basically, the idea is to maximally randomize the distances between each subwoofer and the room boundaries. For example, you might place one near the right corner but several feet up off the ground, and the other about 2/3 of the way back along the left wall at ground level. This spreads out the room interaction peaks and dips.

The second technique is the one I prefer, and it's what I use in my rooms. Very very few recordings have stereo content below 100 Hz or so, limiting the applicability of the Griesinger technique. I have also set up a subwoofer system using the Geddes technique in a modest private recording studio, and the artist is very pleased with the bass definition he can now hear.

Just to be fair, let me say that I did read a paper by someone at Harmon International that if I recall correctly advocated placing dual subwoofers symmetrically in the general vicinity of the main speakers.

The good news is, experimenting with these different placement techniques is inexpensive. You might want to purchase a dolly to save your back if you haven't already, but of course that's optional.

One of the reasons I am asking is that my listening room does not allow to have subwoofers equa-distance from the listening position.

My setup includes mains that will go down to the mid 30s. The only reason for the sub is to add a little bit more info. They are crossed as low as possible, around 40hz.
Then you should be able to put those puppies anywhere they will give you the flattest overall bass response, assuming that is your goal. FWIW I like the idea Duke raised about the Geddes technique and I would try that first. Do you have an SPL meter and a disc w/ 1/3d octave test tones. Sure helps if you do.
As Newbee Stated: A source with 1/3d octave tones is most beneficial when setting up a sub/main system. The Stereophile Test CD 2 contains all three decades. Time alignment is ALWAYS critical. My Tact 2.2X allows placement of the subs in the corners of the room(which unloads them dramatically), delays the mains(time aligns), and equalizes the responses(within .5db). The realism is outstanding.