Cabot does have a weathered gray stain. I think it would be better to just buy the right color. Mixing these two cans might or might not give you the color your after. Plus, if you need more you will have two buy the original mixed stains and you still might not get an exact match.
I think it is best to just buy and use the correct color and save these stains for another project. Also, test the color on a sample pine board before committing to the project.