started gtr at 15.2yr later switch to piano and gone thru classical piano training for 3 yrs.played KB as a pro in bands and solo.and for last 12 yrs playing both(as a solo/band KBdist and blues-rock-hard rock lead guitar player)and .yes GTR knowledge help you play KB and also KB playing and music theory will help you and make you a better guitarist.BTW once you are a guitarist-always a guitarist.there is nothing like giving the vibrato and bending with your bare hands.
and ofcourse in a live band situation GTR player got the babes but KB player got the money and most solo gigs.
that's why Keith (Emerson)said "after choosing the KB I had to go on.people still cheer lead guitar player running around with his 5lb instrument where I would be stuck in a corner(even after those roland AX and yam KX were in the market,real pros won't use them much).later I got rid of the volume hungry guitarists"(KB meg-mid 90's).