I have a Roland KC-200 amp that is 100 watts and it produces all the sound I need in most scenarios for a mono signal for my keyboard. However, I'm starting to play with more synths and keyboards and want more of a stereo spread on my keyboards so I can pan one center left and one center right. Would getting two Vox VX50KB 50 watt apps (or another brand for that matter) and using them as a stereo pair from a small mixer be able to produce the same level of volume as as one 100 watt speaker? Is that how wattage and volume works on speakers?
It's not really an either/or question. Smaller speakers tend not to reach as low as larger ones. However, as soon as you are working with more than one speaker, extinction becomes a thing. A home stereo tends to have a much smaller dedicated space where the speaker sits and you have lots of reflections that fill up possible holes in the sound field. Stage settings aren't as friendly in that respect.
As low frequency speakers grow in size, they tend to become more efficient. However, as they grow in maximum power, they tend to become less efficient again. A difference of just 3dB is equivalent to a doubling of sound power, so actually comparing "50W" to "100W" very much is in the "it depends" category, even more so comparing "2×50W" to "100W".
Often the efficiency of bass speakers determines the efficiency of the setup as the mid/treble speakers can be designed as rather efficient horn speakers.
Instrument speakers particularly in the bass range also tend to have only a part of their maximum power dedicated to linear sound production, so for complex sounds like that of a keyboard there may be a point to either not use speakers to their full capacity or actually divide output between several speaker systems, not based on stereophony but as an actual hard split between reproduced instrument types: in that case, the problem of stereophonic extinction is of course basically a non-issue. This requires keyboards with multiple assignable outputs, however. This can "demuddify" the soundscape of a keyboard somewhat if its speakers are run at significant part of their capacity. In particular separating melodic instruments from bass+drums may help freeing their fundamental frequency reproduction from interference by the bass group.
There will be still auditory masking, of course.
TLDR: don't expect too much particularly in the volume department, but splitting a keyboard's "instruments" between speaker systems can in some cases lead to a more transparent soundscape. This has rather little to do with stereophony, however, and thinking/planning in categories of "stereo" might impact the comparatively modest improvements you can expect to make here.