Multi-driver configurations can cover the frequency spectrum better, but have problems with phase coherence, time-delay, spatial irregularities. Various technical solutions mitigate these, but cannot eliminate them.
Single-drivers have their own advantages and problems: excellent coherence and presence, but limitations in frequency response and their own coherence problems when it comes to complex music.
Coaxial drivers offer a "middle way" that overcomes some of the limitations of both approaches. They must still use a crossover, but it can be simple, and it overcomes the the multi-driver's artificiality of having musical information coming from two or more points in space.
For my ears and budget, coaxials offer the best compromise (remember that all speakers are a compromise) between the virtues of the single-driver and the multi-driver approach.
They are by definition more expensive to engineer and manufacture than two comparable separate drivers.
Try the Great Plains Audio 604-8H-II (www.greatplainsaudio.com) or the Iconic 704-8A (www.iconicspkrs.com) or any of the 12" or 15" Tannoy dual concentrics (I prefer those from the "DMT" series, made from about 1990 to 2002; others swear by the vintage Tannoy models: Monitor Golds, HPDs, etc.)
Another advantage of the coaxials is that is relatively easy to buy raw drivers and crossovers and make, or have made, your own cabinets, which saves serious money compared to buying factory-made cabinets.
Serious Stereo (www.seriousstereo.com) makes a very fine speaker based on the GPA 604-8H-II, but a pair is circa $10K, whereas you can buy the raw drivers direct from the manufacturer for under $1500/pr. and make your own cabinets.
Another advantage of these drivers is a feature they share with many single-driver designs--high sensitivity (95-100 dB), which leaves you free to use low-power SET amps or high powered solid state or anything in between.
BTW, I have no affiliation with any of the companies I mentioned; I'm just a long-time and satisfied user of some of their products.
Happy listening!
Joel.