Interesting question on 2 fronts: How you can and why you would want to do it.
First the how. This is not as easy as it first appears since as far as I know there is no direct bus beween La Plagne and Bourg st Maurice. You will have to get the bus to Aime (Autocars Martin?) then the train to Boug st Maurice and then the funicular to Arc 1600 and reverse it all at the end of the day.
It is much much easier to get the Vanoise Express across. Though I may be wrong this may be possible using nothing harder than blue pistes.
Now the why. The skiing in Les Arcs is similar to that in La Plagne, but with more harder runs. Even its greatest fans (and I am one) would not consider a worthy tourist destination apart from the skiing and other mountain activities. I cannot think of anythig you can get in Les Arcs that you cannot get in La Plagne.
Sorry I have not been a lot of help
John