as Nathan Wolek's toolkit is non-native (external) to Max, you are assuming that people reading this have it. And many do, but many don't. I don't - so why am I replying :)
To turn a mono source like this into stereo one, you need to get inside the grain engine, but as an external the contents of 'grain.play' may not be editable. Simply duplicating the output to both channels is, as you know, not stereo. If you CAN hack Nathan's grain engine (ie open the grain.play object), you need to duplicate the [play~] object and all associated windowing and output - a process too complicated to describe or illustrate here. Sorry not to be more help, but this tutorial of mine may help illustrate the concept:
The good news is that gran.groove.file and gran.play.file are not externals - they're abstractions, so you can edit them.
That said there are some custom externals within, like gran.phase~ and it doesn't appear that you can set the channel it accesses, so you would have to split your stereo files into two mono files and load into separate buffers, but it can be done,