I am not sure what the problem is, exactly. Does the -6 channelling give you too much channelling? I mean, above 99% so that in fact ur channelling becomes slower?
It is the added effect of having two pieces of gear from the same 'brand' so to speak. If u do not have any other channellling stones or gears, the effect is zero. It would be weird for a cleric not to have instant channelling, but ok.
If you are over 99% due to these stacked stats, remove a channelling stone from one of ur gears and the problem should be solved. If u get more gears from this line, you may get even more - channelling so may have to remove more stones.
If this does not answer you problem then plz be more specific about the exact problem.