http://en.wikipedia.org/wiki/Ammonium_thiosulfate

http://en.wikipedia.org/wiki/Sodium_thiosulfate

Based on what I read at the above links, there should be no problem mixing them until you can dump them. I would recommend a trial with a small amount of each in a plastic bottle.

As you can see, Ammonium thiosulfate is basically non-toxic, and can even be used as fertilizer. Sodium thiosulfate is even used in medicine, as a disinfectant and chelating agent, and is even used in table salt. Even though the laws may discourage dumping, it is safer than many other things we put into the ground.