First off, you don't have to set ty, tz or rx, as you're not changing anything.

The easiest way to do this would be to use two shelf buttons or hotkeys with the names changed. There is another...

You could also take a look at the Mirror Animation tool provided in Maya Bonus Tools. (Separate download)

One way to multiply the values by -1 would be:

setAttr("pCube1.tx", getAttr("pCube1.tx") * -1);
setAttr("pCube1.ry", getAttr("pCube1.ry") * -1);

- and + can be found in the Hotkey Editor under...