Here is a link for purchasaing dual cartridges. Also, you could attach a static mixer to the cartridge, so as the material comes out it is mixed. The only drawback is that you waste all the material that is left in the static mixer.
I know that it is measured by voluume, I wonder if it were measured by weight, how close you would be?
http://www.staticmixerdispenser.com/main/cartridges.html