I think i knew you could make a special backpack by combining various amulets with it but i must have forgotten about it till today and i combined an amulet of talos with my backpack and im wondering does the 20% less time till you shout stack with the shrines of talos blessings?

Good question. If it works as an amulet, it should stack with the blessing, http://elderscrolls.wikia.com/wiki/Talk:Amulet_of_Talos. I wonder if the amulet on the backpack will stack with an ordinary amulet?

It's not an additional 20%, backpack charms are between 1/3 and 1/2 original strength, do not stack with another amulet but do stack with shrine blessings.