I don't agree entirely with your assessment, curegeorg. One of the major reasons to have 2 subs besides just bass reinforcement, is to even out the room response. Which two subs at good positions can do quite nicely.
Locating 1 sub is more difficult. But if you have one sub already at an optimum location, the addition of another can only help, unless it's located in a really dead zone.
You would probably want them in asymmetrical positions as not to chance further augmentation of any adverse room response. And being such low frequencies and basically non-directional, I don't think not being the same type is critical at all.
I hate to think of tb27272 giving up an M&K350, which a beaut of a sub, and I'm not sure he would gain that much with 3, for the added cost. Granted it would have more to do with the rest of his speaker system.

For just bass reinforcement, assuming the prime location is the best, one can even stack one on top of another for just 'more' bass. For that specific application, I think identical subs 'might' be warranted.
But in different room locations, I really doubt it will make a difference of any significance.
But I admit having limited hands-on experience in this area, and am basing it on where I have experimented with different room acoustics.