I do not have normal use and dexterity in my left hand which prevents me from being able to twist the grip shifter on the left bar of my bike.  I am  looking for ideas, but also would like to know if it would be possible to put a single trigger sifter on the right bar (to control the front derailleur) so I could shift gears on the front chain ring with my right hand.
 
I appreciate any input or ideas you all may have.
 
Thanks in advance,
Mikep

Mike I too have issues in both hands.  A half pipe shifter is worthless to me, and MOST bent come with those.  The long pipe shifters from SRAM work SO much easier.  I only just discovered that they are still made because my wife's bike came with them.  Those might be worth a shot, if you'd rather stick to twisties.
 
But there's no reason why you shouldn't be able to change to having different shifters front and rear - I know Terry Pickl did it on his baron at one time.
 
You could also try bar ends if you like.  Those have worked well for me.  
 
 

Actually your best bet would be trigger rear with grip shift for the front on the right side of the bar, with the front shifter mounted upside-down. This would get the cables clear of each other as well as using the proper shifter for each derailler.Your alternative would be a friction shifter on the bar for the front derailler with your existing grip shifter for the rear.