I would strongly suggest not placing a stereo pair of drivers in the same (un-partitioned) enclosure for either bass or midrange purposes.
The biggest problem is bass - it would only work acceptably if the bass signal driving both left and right was identical (eg a mono bass signal) and while it's fairly common in music for most bass notes/instruments to be located in phase in the centre "phantom" channel, it's by no means universal.
Some Jazz music puts the bass instrument well off to one side, while I have also come across some music that deliberately puts the bass out of phase and unequal in amplitude.
The amount of loading each woofer will see when both share the same enclosure depends on what the other woofer is doing.
If both woofers are being driven by exactly the same signal, the effect on the bass alignment and loading of each individual woofer is exactly the same as if the box was partitioned down the middle with each woofer working in half the available space.
In other words if it's a 50 litre box with both drivers in the same 50 litres, when driven equally in phase each driver effectively only "sees" 25 litres, regardless of whether the box is partitioned or not. This is a very important point - you don't magically gain any additional effective volume by not partitioning the box, except a small amount eliminating the lost volume consumed by the partition thickness.
On the other hand, if only one channel is being driven by the bass (by the bass instrument being fully left or right for example) then one driver "sees" the whole 50 litres and the bass alignment will completely change compared to both drivers being driven.
(If the second driver was disconnected it would act like a passive radiator, but if still connected and "controlled" by the low output impedance of the amplifier, it will tend to just act as a small additional compliance)
For example if it was a critically aligned bass reflex enclosure with both drivers being driven, it would be an under-damped boom box with a large peak in the low frequency response with only one driver being driven, because you've effectively doubled the box size.
Dramatically increasing the effective box size like this will also make it much more prone to being over driven at low frequencies.
Worse still, if you feed both speakers out-of-phase bass, both speakers will see "infinite" box volume and will have no loading whatsoever, putting the drivers at severe risk of being over-driven with even relatively small drive levels.
Sure, out of phase bass is rare in music, but do you want that one piece of music to destroy your woofers ?
Even with more normal bass where you might have a kick drum part way to one side of the L-R stage and the bass guitar part way to the other side, the unequal drive between the two drivers is going to cause all kinds of unnecessary intermodulation distortion effects in the bass. (One woofers excursion will physically modulate the other woofer cone position etc)
There's absolutely no good reason to do any of this though - if you must use the same cabinet simply partition the inside of the box in half so each woofer is in their own enclosure - apart from a very small percentage of lost volume from the partition panel thickness the bass alignment when reproducing mono bass will be almost identical to not having the partition, and in all other circ*mstances (offset bass, reverse phase bass) there will be no bad side effects, unlike the case where they share an enclosure but aren't driven by the same signal.
In the case of midrange I still wouldn't put stereo midrange drivers in the same enclosure - a surprising amount of the midrange that comes through the front of a midrange cone has already bounced around inside the enclosure and passed back through the cone - with two drivers in the same enclosure unless you had a ton of damping material you'd get a lot of mixing of the back-wave from each driver re-radiating through BOTH cones, messing up stereo imaging.
If it was a dedicated midrange enclosure (in a 3 way for example) you have the added problem that the mechanical resonance of the driver/enclosure is up in the lower midrange 150-300Hz region, and you'll get all the same problems as with bass in a 2 way where the response roll-off changes depending on whether both drivers are driven or not, or in phase or not.
And unlike bass, midrange often IS out of phase (for surround effects) and is very often off to one side.
In short, don't do it - just partition the cabinet and be done with it 
