Dual rear tire valves

I just bought my first dual rear tire truck and am wondering about the inner tire's inflation. I can't get my hand into the small opening in the wheel to reach the valve stem. Is there a tool to reach the stem and remove the cap? I am also looking at a valve extension but am concerned that the centrifugal force of the spinning wheel will exert undo force on the extension and possibly cause a problem with the valve stem. What is the usual solution to this problem? Thanks for any help on this.