All things being equal except impedience, 8 ohns Vs. two 4 ohm speakers.
Speaker impedance isn't constant -- it varies widely with the
frequency and is dependant on the enclosure too. For two
identical speakers in identical enclosures, it should be OK.
For two different speakers with different impedance/frequency
graphs, they would each sound like they were being driven
through different graphics equalizers with the sliders set to
random positions.
You would get more predictable results with two 16 ohm speakers
in parallel. Then they wouldn't need to be identically matched.
In either case, the total sound output might be higher or lower.
That would depend on the efficiency of the two speakers verses
the original speaker. The output can become much more directional
though (Youngs Slits effect*) giving loud and quiet areas, which
in some cases (off-centre) will vary by frequency.
*See
http://www.practicalphysics.org/go/Experiment_115.html