Pfft ........ 
 
 
...... nuttin' wrong with my brane  

Mesh
No thanks....not interested in your scientology/cosmology.

+1

Not just guitar players, and probably not just musicians. 
Think about it. The human nervous system is demonstrably incapable of the speed necessary to use a feedback loop to synchronize ensemble playing. In other words your brain and muscle control are not fast enough to hear what another player is playing and match/complement his action in real time. There would be a very noticeable delay in your response. Even with an infinitely fast response time, the distance between players on a large stage and the limit of the speed of sound, although small compared to the nervous system latency, would be greater than most people could tolerate from a DAW. When players are ensemble, in the groove, they are constantly playing what they anticipate their fellow player will produce and synchronizing their internal clocks in order to make what appears to be coordinated play. It is actually independent play of consistent performance simultaneously. There is absolutely no way that you could add into this neurologic latency any conscious consideration and choice based on what you are hearing. Consciousness can guide your internal processing, by anticipating the direction you both will follow, but it is far to slow to be the mechanism for coordination. Try playing a totally "wrong" chord, and your partner will not hit it at the same time or even very close even if he is trying to "follow" you. 

tlw
An article stating the very obvious about any competent musician capable of playing with others.

Equally applicable to any group activity relying on synchronising different people to do things together I would have thought. And switching off conscious thinking about the minutia of playing an instrument and going with muscle memory and rehearsed and ingrained habits applies to good instrumentalists, singers, typists, driving......

After all, all that practice is intended to get us to the point where we can play without thinking "press 1st finger left hand down behind third fret on 2nd string, pluck second string with plectrum, now press 3rd finger behind 4th fret on 3rd string, pluck 3rd string, bend string upwards a bit" all the time. And that applies to any instrument.

Nothing to do with whether someone learns material from sheet music or entirely by ear either, the mechanics are the same other than sheet music saves time working out what the notes are, though for many genres it doesn't convey style or 'feel' very well. And anyway, many musicians do both.

What I'd like to know is why I can co-ordinate my hands perfectly well enough to play guitar, bass, a few other stringed instruments and some of the accordion family but I'm hopeless at two-handed keyboard playing.

"Practice?" 

slartabartfast
Not just guitar players, and probably not just musicians. 
Think about it. The human nervous system is demonstrably incapable of the speed necessary to use a feedback loop to synchronize ensemble playing. In other words your brain and muscle control are not fast enough to hear what another player is playing and match/complement his action in real time. There would be a very noticeable delay in your response. Even with an infinitely fast response time, the distance between players on a large stage and the limit of the speed of sound, although small compared to the nervous system latency, would be greater than most people could tolerate from a DAW. When players are ensemble, in the groove, they are constantly playing what they anticipate their fellow player will produce and synchronizing their internal clocks in order to make what appears to be coordinated play. It is actually independent play of consistent performance simultaneously. There is absolutely no way that you could add into this neurologic latency any conscious consideration and choice based on what you are hearing. Consciousness can guide your internal processing, by anticipating the direction you both will follow, but it is far to slow to be the mechanism for coordination. Try playing a totally "wrong" chord, and your partner will not hit it at the same time or even very close even if he is trying to "follow" you. 

Hey, Like a look ahead plug in! Right! 
 
I think much of it is in knowing another player and their style. This lets you pre think what they are likely to do next.
Sometimes a simple facial expression can be enough of a guide if you know them. Knowing the key is important even if you aren't reading music. I wouldn't include covers here. If they have the kind of personality that likes to change keys mid song simply slap them silly.
 
I can usually stay in the pocket playing a secondary instrument. For me it's more about staying out of the way. Two leads, not so much. Put them in a room together and let one of them kill the other one.
 
Playing without music can be really easy. If playing on piano simply pick a simple key like C or G. Works for me every time.