if you want to use the version of MSO on the Surface RT you have to purchase a separate commercial MSO licence for it and only it.

As to running two apps at once, I do a heck of a lot of story writing and editing and the office package I use is set for a book size of 6 x 9 inches and to display that only on the screen properly needs a screen with an 18 inch diagonal, so I don't see how you can get two apps properly displayed on the Surface screen unless they're very small.