Bass "tightness" has nothing to do with driver size. A properly designed 21" sub driver is no less "tight sounding" than an 8" driver while playing within it's rated frequencies with the proper power behind it. Actually, it can be argued that the opposite would be true since the larger driver would have to move considerably less than the smaller driver to produce the same frequency at the same dB.
Mixing sub drivers in a single enclosure is not done because each driver has different T/S parameters which determine the box size and tuning in the case of ported, bandpass or horn loaded enclosures. You cannot simple throw two different drivers in a box and hope that crossovers give you a good response. The best you can do with multiple drivers sharing a common enclosure is to use the exact same drivers. Even two same sized drivers from different manufacturers or even the same manufacturer, but different models will not work. The only way to use multiple drivers in "one" enclosure is to separate the drivers from each other. Essentially you are building two separate boxes that share a common wall. Think of it like an apartment building. Two separate apartments sharing a common wall. Each does its own thing independent of the other. At that point you might as well build two individual boxes to take advantage of room modes.
Again, best thing to do is get a properly designed driver in the size and specifications you need and build a subwoofer enclosure around it. I can tell you my 18" LMS has a lot "tighter" bass than my cousin's 8" Pioneer.