What your going to have to do is to run the line from the toilet to the main line. Since this is a branch line off the main to your toilet, the connection between the toilet and the sink can have a clog downline, but uphill from the main cleanout. Every fixture that goes to the sewer is on a branch, very much like the branches of a tree; each one connects into the trunk. Just after the point where the two branches come together heading to the trunk line is the clog. What your going to have to do is to pull the toilet off the floor and clean the line down the hole in the floor for the toilet. This will guarantee you reach the clog thats affecting both fixtures. Since you cannot (most of the time unless your good at it) go backwards up the line from the cleanout, you can not reach the point of the clog. In addition you probably would not want to do that, as you can break your toilet with a main line snake. In addition since the piping is getting smaller the further uphill into the system you go, you can get stuck with your snake. Not a good proposition.