They were separate companies until about five years ago when TDAmeritrade acquired thinkorswim. There is a lot of duplication and redundancy between the two platforms, tos and Trade Architect, but, for whatever reason, TDA has decided to leave both platforms as they are. I am aware of tos clients, who were grandfathered into TDA upon the acquisition, who have never used Trade Architect and who probably never will.

I trade options in addition to stocks and I use tos for my options trading. I also use their Level 2 for my stocks even though I rarely trade stocks through tos (I use Etrade for most of my stock trading).