Can I fit shimano ST6600 lever with shimano tourney 105 gear?
Can I fit the dual control levers with the the shimano tourney 105 gear?
I think you may be a bit confused. Shimano Tourney and Shimano 105 are 2 totally different systems. Tourney is used on very cheap bikes while 105 is a midrange road bike gruppo.
ST6600 is Shimano Ultegra which will work with 105 but NOT with Tourney. Now, if you want dual control levers I THINK that you can use Shimano Sora (ST2200) for Tourney, but don’t expect any improvement in performance. Also, you need to make absolutely sure that if you currently have friction shifters that the freewheel on your bike will match up with the shifters… some freewheels are of very poor quality and don’t have the precision necessary to use indexed shifters.