I think your logic is sound in that two subs placed very close together will have similar peaks/valleys in bass response. The slight difference in position could make a difference, but I'd guess it would be minimal.

The manufacturer of my sub suggests that multiple subs be placed together. I'm sure they know more about this than I do, but if you have a major room boundary issue (or suckout), it seems more logical to have the subs in separate locations in order to even out room pressurization. Some publications suggest that this is also the way to go with multiple subs (and personal experience also seems to support this). It makes sense, really. If you're pressurizing the room from different locations, boundary effects will be unique for each sub, giving a more even response - assuming you take time to find the best locations. Of course, then you may end up with timing issues if the subs are not equal distant to the listening area.

I know I could never place another sub in the corner with my existing one. The HUGE peak at 30Hz would be untamable with two subs there (built-in PEQ can barely tame it as it is). Luckily, I have no real suckout zones, though I certainly have a few areas that do have stronger bass response, overall. Two subs, properly placed, would likely even things out a bit.