I think the only time your are presented with the option is on the launcher install. That won't help if you only want to use one launcher. Someone else said to install another instance of the launcher on another drive and install PTS that way. I may give that a try if no one else has any other solution.
For those wondering on this...it looks like installing another launcher to a different drive works. I'm downloading PTS now to another drive, yet my original install of live is untouched. You just have to have two shortcuts, one for the live launcher and one for the PTS launcher. (Thanks to mra2414 who posted this in the main PTS section!)