Probably more problematic than the inequity of distance and arrival time for multiple subwoofer locations are the conflicts caused by different loading methods.
An acoustic circuit has all the elements of an electrical circuit. Mass and compliance equate to inductance and capacitance. Changes in subwoofer loading result in non-linear changes in the phase response.
Typically when you're seeking secondary locations you're having to load the individual subwoofers in an asymmetrical manner. And usually the secondary locations are a compromise and less effective than the primary location.
How a woofer loads, ie: sealed, ported, bandpass, direct-radiating, down-firing, vented from within an enclosed locker, helm console or bench seat, creates different phase characteristics. For example, a subwoofer mounted in a port locker, even though it's vented, has energy that is stored and released differently than a direct-radiating subwoofer. The phase response varies across the spectrum. The two different loading methods don't combine well. The sum doeasn't equal the parts the same as symmetrically mounted and loaded subwoofers.
Reflective boundaries, reinforcing planes, locker displacements and more have an impact on the amplitude response which has an effect on the phase response. The two are inseparable.
Also, its better to have a little excess power for your woofer. Having more woofers or woofer mass than your amplifier can fully control is counterproductive. It's better to have fewer woofers and maintain the right power to mass ratio than it is to dilute your power into multiple woofers that may be in some degree of conflict.
For those who have added additional subs in different locations and were a little disappointed with the results, hopefully this gives some explanation as to why.
With enough subs and power you can have enough leverage to overcome just about anything. It may sound a little muddy and may not be the most effective way to proceed.
Multiple subs mounted in the same location and mounted symmetrically relating to their boundaries, combine very will with no downside.
So I would recommend a single sub, the best you can afford, with lots of power, in the most optimum location.