I tend to agree with Duke - the electrical aspect of wiring up two woofers is not such a big deal.
To me the issue is the problem of precise driver matching and the backwave acoustic coupling in the box and the fact that two small woofers with two small voice coils rarely perform like a big bad boy woofer with a massive 4 inch VC. For starters the big VC is going to dissipate heat much more easily due to the larger surface area. The other issue is lobing - by separating the drivers spatially then you run into this problem at lower frequencies than a single driver - this can cause a reduction in reflected energy at certain specific frequencies.
To me the issue is the problem of precise driver matching and the backwave acoustic coupling in the box and the fact that two small woofers with two small voice coils rarely perform like a big bad boy woofer with a massive 4 inch VC. For starters the big VC is going to dissipate heat much more easily due to the larger surface area. The other issue is lobing - by separating the drivers spatially then you run into this problem at lower frequencies than a single driver - this can cause a reduction in reflected energy at certain specific frequencies.