First, are you having only one viewer? In any setup, only one line of viewers (who would likely be in each other's way) can be equidistant from both subwoofers even if the subwoofers were perfectly symmetrically placed. If the subs are not symmetrically placed, there will still be places in the room where the distance from each sub will be the same.
Second, perfectly symmetrical subwoofers are rarely the perfect way to go. If placed perfectly symmetrically in a symmetrical room, they both will be introducing the same set of standing waves. Unless you have a good deal of acoustic room treatment meant to prevent bass-frequency standing waves, creating standing waves that reinforce each other is likely much less than ideal.
Third, there is so much bass-frequency energy in a room that arrives at listening positions from other than a direct line from two subs that significant acoustic bass energy is ‘arriving from almost everywhere’ and is far from time-coherent.
Fourth, if the speed of sound in air is about 1100 feet per second, and the highest used frequencies of your sub(s) is about 100 Hz, the shortest wavelength coming from your subs is about 11 feet. If one sub is a foot closer or further from a listener than the other sub, that is still less than 10% of the shortest wavelength, and the two subs will be very much more time-coherent compared to each other than the rest of the large bass room response will be compared to the original radiation from the subs. If a non-symmetrical placement means that one sub is 9 feet from its closest listener and 13 feet from its most distant listener, while the other sub is 11 feet and 15 feet respectively, the average distance is about 12 feet and would likely do well as a subwoofer distance setting.
One can argue the validity of these points, but unless you are moving into a nearly ideal environment, trying to make sure that each sub is within an inch of being symmetrically placed and equidistant from a listening position is straining at a gnat while swallowing a camel.
In my situation I have experimented with ‘stereo sub’ placement and found that, in my room, a good mix of results is obtained by placing one sub 6 to 10 inches from the side wall and three to four feet from the back wall, while the other sub is offset at about the same distances but the offsets are swapped, 6 to 10 inches from the back wall and three to four feet from the side wall. This gives each sub similar near-field (wall and corner) acoustic support but also means that each sub will not be feeding the same standing wave placement in the same exact way either side-to-side or end-to-end in the room. Small variances in placement, even two inches or so, can result in an overall smoother, or conversely a more uneven, distribution of whatever standing waves do resonate in your listening environment. Pick two or three liked and well-known sections of music or movie where bass detail matters and try various placements, first moving each sub a foot or two between tests, then later reducing the changes to just inches at a time. Be sure to listen from all your listening positions.
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